Inkjet printing methods and inkjet ink sets

ABSTRACT

An inkjet printing method including, in order, the steps of: a) providing at least two or more color inkjet inks of the same color and color density but having a different composition to an inkjet printer; b) mixing the two or more color inkjet inks in a controlled amount; and c) printing the mixture of the two or more color inkjet inks with the inkjet printer onto an ink-receiver. An inkjet printer and an inkjet ink set include two or more color inkjet inks of the same color and color density but having a different composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2007/062708, filed Nov. 22, 2007. This application claims thebenefit of U.S. Provisional Application No. 60/885,493, filed Jan. 18,2007, which is incorporated by reference herein in its entirety. Inaddition, this application claims the benefit of European ApplicationNo. 06126887.6, filed Dec. 21, 2006, which is also incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to inkjet printing methods and inkjet inksets wherein the inks are jetted upon different types of ink-receivers.

2. Description of the Related Art

In inkjet printing, tiny drops of ink fluid are projected directly ontoan ink-receiver surface without physical contact between the printingdevice and the ink-receiver. The printing device stores the printingdata electronically and controls a mechanism for ejecting the dropsimage-wise. Printing is accomplished by moving a print head across theink-receiver or vice versa or both.

When jetting the inkjet ink onto an ink-receiver, the ink typicallyincludes a liquid vehicle and one or more solids, such as dyes orpigments and polymeric binders. It will be readily understood that theoptimal composition of such ink is dependent on the printing method usedand on the nature of the ink-receiver to be printed. The inkcompositions can be roughly divided in:

-   -   water-based, the drying mechanism involving absorption,        penetration and evaporation;    -   solvent-based, the drying primarily involving evaporation;    -   oil-based, the drying involving absorption and penetration;    -   hot melt or phase change, in which the ink is liquid at the        ejection temperature but solid at room temperature and wherein        drying is replaced by solidification; and    -   UV-curable, in which drying is replaced by polymerization.

It should be clear that the first three types of ink compositions aremore suitable for a receiving medium that is more or less absorptive,whereas hot melt inks and UV-curable inks are usually printed onnon-absorbent ink-receivers.

However the behavior and interaction of a UV-curable ink on asubstantially non-absorbing ink-receiver was found to be quitecomplicated compared to water-based inks on absorbent ink-receivers. Inparticular, a good and controlled spreading of the ink on theink-receiver proved to be problematic and adhesion problems weresometimes observed on using different types of non-absorbingink-receivers. The same problems have been observed when solvent basedinkjet inks including a binder were jetted on different types ofnon-absorbing ink-receivers.

One way to approach these problems is to develop and use different inksets for different types of substrates, but this is a not a preferredsolution since changing inks in the printer and print head is very timeconsuming and not really a viable solution for an industrial printingenvironment. Therefore, the general approach is to modify the surfacechemistry of the ink-receiver either with a suitable surface layercoating or by a pre-treatment such as plasma or corona treatment.

Corona discharge treatment and plasma treatment increase the cost,complexity and maintenance of the equipment used to process thesubstrates. Substrates may contain significant impurities orirregularities that may interfere with the treatment of the substrate,and hence not result to the uniform spreading and adhesion of ink.

The other possibility of using the same inkjet ink set on differentink-receivers by application of a surface layer prior to jetting alsoincreases the complexity of the inkjet printer. Generally, the surfacelayer is coated and dried or cured before jetting the inkjet ink as, forexample, in the inkjet printing process in EP 1671805 A (AGFA) and US2003021961 (3M), but it can also remain a wet, un-cured surface layer asin WO 00/30856 (XAAR).

A single composition of a surface layer suitable for all the differentsubstrates is however not available. WO 2006/111707 (SUN CHEMICAL)discloses a process of ink jet printing in which: i) a primer is appliedto a substrate material; ii) ink is ink jet printed onto the primedsubstrate; iii) a characteristic relating to print quality is evaluated;iv) the composition of the primer is adjusted in dependence on theevaluated characteristic relating to print quality; and v) the adjustedprimer composition is applied to the substrate material and ink is inkjet printed onto the primed substrate material to give a printedproduct. Surface layers substantially increase the thickness of anink-layer, which may result in a different look-and-feel and reducedflexibility of the ink-layer.

Inkjet printing methods wherein inkjet inks are mixed with colorlessliquids or other color inks just prior to jetting have also beeninvestigated.

U.S. Pat. No. 6,550,892 (KODAK) discloses a drop-on-demand ink jetprinting system for delivering droplets of selectable-color ink to areceiver by mixing a colorless liquid ink with liquid inks of adifferent color and delivering the ink mixture to the ejection chamberof a print head. Also U.S. Pat. No. 6,050,680 (CANON) relates to an inkjet recording apparatus that can record images with a plurality of inkswith different densities for each color by mixing of a first inkcontaining colorant and a second ink containing no colorant.

U.S. Pat. No. 6,097,406 (KODAK) discloses an inkjet printing apparatusfor printing continuous tone images wherein a plurality of colorants orcolorants precursor are mixed to produce a desired colorant.

Instead of mixing colored inks, U.S. Pat. No. 4,614,953 (LAITRAM)discloses a color ink jet printing mechanism utilizing a single streamflow of ink by injecting solid dyes into a carrier fluid to form coloredink. The mechanism is capable of a wider range of color tonalities, dueto the premixing capabilities, than is possible using ditheringtechniques with three colored inks.

U.S. Pat. No. 5,854,642 (CANON) discloses ink bottles containing inkshaving the same tone and different compositions adapted to respectivekinds of fibers for a cloth are provided. The inks differ in compositionby the type of colorant used.

It would be desirable to be able to print inkjet inks with consistentimage quality on a wide variety of ink-receivers using astate-of-the-art inkjet printer not requiring any complex or costlyadaptation of the printer.

Printing on a wide variety of different ink-receivers, includingnon-absorbing substrates such as glass, metal or polymeric surfaces,delivers frequently inconsistent image quality and/or adhesion problemsof the ink to the ink-receivers. A change of substrate oftennecessitates a cumbersome change of inkjet ink sets, a second inkjetprinter or some pre-treatment of the substrate, which are all notdesirable for reasons of productivity.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide inkjet ink sets and inkjet printingmethods capable of handling a wide variety of different types ofsubstrates using a state-of-the-art inkjet printer without compromisingon the consistency of image quality, physical properties, such asadhesion of the image to the substrate, and productivity.

Further preferred embodiments of the present invention will becomeapparent from the description hereinafter.

The mixing of inkjet inks with colorless liquids and other color inksjust prior to jetting may cause changes in the colorant concentrationdue to less accurate mixing leading to differences in color gamut orimage quality if no time-consuming adaptations to the color managementof the inkjet printer are made.

It has been found that through mixing inks containing the samecolorant(s) in approximately the same concentration but wherein theremaining part of the inks, i.e. the so-called ink carrier, was of awell-chosen different composition that images could be producedexhibiting the desired spreading and/or adhesion and image quality on awide variety of substrates without the need to adjust the colormanagement of the inkjet printer.

Another preferred embodiment of the present invention includes an inkjetprinting method including in order the steps of:

a) providing at least two or more color inkjet inks of the same colorand color density but having a different composition to an inkjetprinter;b) mixing the two or more color inkjet inks in a controlled amount; andc) printing the mixture of the two or more color inkjet inks with theinkjet printer onto an ink-receiver.

Another preferred embodiment of the present invention includes an inkjetink set including two or more color inkjet inks of the same color andcolor density but having a different composition.

In radiation curable inkjet printing, the method of mixing color inkjetinks of the same color and color density was advantageously used forobtaining higher curing speeds. The stability of the color inkjet inkswas improved by incorporating the photo-initiator in one ink and theco-initiator in another color ink of the same color. Inkjet ink mixturescould be prepared exhibiting higher curing speed by incorporating higherconcentrations of photo-initiator and polymerization synergist in thedifferent color inkjet inks of the same color and color density.

The mixing of two or more color inkjet inks of the same color and colordensity can be advantageously exploited for many purposes which mayrelate to:

-   -   image quality, e.g. dotsize, gloss, line quality and bleeding;    -   physical properties of the ink, e.g. viscosity, temperature,        shelf-life stability, surface tension, drying time, curing        speed, adhesion to a substrate, flexibility and hardness of an        ink layer; and    -   jetting performance of the printer, e.g. latency, pooling of the        nozzle plate, failing nozzles, drop formation, and satellite        formation.

Differences in gloss between the inkjet inks and the substrate usuallylead to mediocre image quality. By mixing, in an appropriate ratio, thecorresponding color inks resulting in high respectively low glossyprints, the gloss of the inkjet inks can be matched with that of aspecific substrate resulting in improved image quality. For a secondsubstrate having a different gloss value, another ratio of the inkjetink with a high gloss value and the inkjet ink with a low gloss value isthen chosen.

The mixing of two or more color inkjet inks of the same color and colordensity just prior to jetting can also be advantageously exploited toinclude security features for security documents. Usually a plain colorink is then mixed in a well chosen ratio with another color inkincluding a fluorescent compound, a phosphorescent compound, athermochromic compound, an iridescent compound or magnetic particles.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are merely examples of possible constructions formixing the color inkjet inks of the same color and color density in orat the inkjet printer.

FIG. 1 is a schematic view of a system supplying a first ink “Ink 1” tothe inkjet print head via a conduit wherein a second ink “Ink 2” andconsecutively a third ink “Ink 3” are added in a controlled amount.

FIG. 2 is a schematic view of a system supplying a first ink “Ink 1” tothe inkjet print head via a conduit wherein a mixture of a second ink“Ink 2” and a third ink “Ink 3” is added in a controlled amount.

FIG. 3 is a schematic view of a system supplying in a controlled amounta first ink “Ink 1” and a second ink “Ink 2” to an ink mixing chamberwhich then delivers the ink mixture to the inkjet print head.

FIG. 4 is a schematic view of a system supplying in a controlled amounta first ink “Ink 1” and a second ink “Ink 2” to an ink mixing chamberwhich delivers the ink mixture to the inkjet print head via a conduitwherein a third ink “Ink 3” is added in a controlled amount.

FIG. 5 is a schematic view of a system supplying a first ink “Ink 1”, asecond ink “Ink 2” and a third ink “Ink 3” in a controlled amount to anink mix chamber (not shown) incorporated in the inkjet print head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term “inkjet ink set”, as used in disclosing the present inventionmeans the inkjet ink set which is coupled to the inkjet printer. It can,for example, be made up from two separate commercially available CMYKinkjet ink sets each including four inkjet inks C, M, Y and K, as longas the inkjet inks of the same color from both CMYK inkjet ink setsfulfill the requirements of the present invention. Alternatively it isalso possible to use a CC′MM′YY′KK′ inkjet ink set including two cyaninks C and C′, two magenta inks M and M′, two yellow inks Y and Y′ andtwo black inks K and K′, which is in accordance with the presentinvention.

The term “colorant”, as used in disclosing the present invention meansdyes and pigments.

The term “dye”, as used in disclosing the present invention means acolorant having a solubility of 10 mg/L or more in the medium in whichit is applied and under the ambient conditions pertaining.

The term “pigment” is defined in DIN 55943, herein incorporated byreference, as a coloring agent that is practically insoluble in theapplication medium under the pertaining ambient conditions, hence havinga solubility of less than 10 mg/L therein.

The term “C.I.” is used in disclosing the present application as anabbreviation for Color Index.

The term “UV” is used in disclosing the present invention as anabbreviation for ultraviolet radiation.

The term “ultraviolet radiation” as used in disclosing the presentinvention means electromagnetic radiation in the wavelength range of 100to 400 nanometers.

The term “wt %” is used in disclosing the present invention as anabbreviation for % by weight based upon the total weight of the inkunless otherwise specified.

The term “actinic radiation” as used in disclosing the present inventionmeans electromagnetic radiation capable of initiating photochemicalreactions.

The term “Norrish Type I initiator” as used in disclosing the presentinvention, means an initiator which cleaves after excitation, yieldingthe initiating radical immediately.

The term “Norrish Type II initiator” as used in disclosing the presentinvention, means an initiator which in its excited state forms freeradicals by hydrogen abstraction or electron extraction from a secondcompound that becomes the actual initiating free radical. The secondcompound is called co-initiator or polymerization synergist. Synergistsare compounds having a carbon atom with at least one hydrogen atom inthe α-position to a nitrogen atom.

The term “photo-acid generator” as used in disclosing the presentinvention means an initiator, which generates an acid or hemi-acid uponexposure to actinic radiation. A photo-acid generator is often alsocalled a cationic initiator.

The term “thermal initiator” as used in disclosing the present inventionmeans an initiator, which generates initiating species upon exposure toheat.

The term “alkyl” means all variants possible for each number of carbonatoms in the alkyl group i.e. for three carbon atoms: n-propyl andisopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl;for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyland 2-methyl-butyl etc.

Inkjet Printing Methods

The inkjet printing method including in order the steps of:

a) providing at least two or more color inkjet inks of the same colorand color density but having a different composition to an inkjetprinter;b) mixing the two or more color inkjet inks in a controlled amount; andc) printing the mixture of the two or more color inkjet inks with theinkjet printer onto an ink-receiver.

Although the possibilities of adapting the ink mixture to a specificink-receiver increases with the number of color inkjet inks of the samecolor and color density present in an ink set, many problems of imagequality consistency and adhesion on different substrates can already besolved by using only two color inkjet inks of the same color and colordensity.

In another embodiment, the inkjet ink set includes three color inkjetinks of the same color and color density for handling all the differentsubstrates.

In the most preferred embodiment, the ink set includes two or more colorinkjet inks of the same color and color density for each of the colors,e.g. cyan, magenta, yellow and black color, wherein the differentlycolored inks have corresponding compositions of solids and liquidcomponents. For example, with the exception of the colorants, the samesolids and solvents of a first yellow ink can be found in approximatelythe same amounts in a corresponding first cyan ink, while the differentsolids and solvents of a second yellow ink are also found inapproximately the same amounts in a corresponding second cyan ink.

In another embodiment, the inkjet printing method uses a so-called“multi-density” inkjet ink set including two or more color inkjet inksof the same color and color density and two or more color inkjet inks ofthe same color but a smaller color density. For example, the ink set mayinclude two or more “dark magenta” inkjet inks of a same color densityand also two or more “light magenta” inkjet inks of a same color densitybut smaller than the color density of the “dark magenta” inkjet inks. Inanother preferred embodiment the multi-density inkjet ink set includesdark and light inkjet inks for the colors magenta and cyan. Dark blackand light black inks may also be present in an inkjet ink set.

The inkjet printing method according to a preferred embodiment of thepresent invention preferably uses an inkjet ink set wherein the two ormore color inkjet inks of the same color but a smaller color densitycontain the same type of colorants, most preferably the colorants areidentical.

In the inkjet printing method according a preferred embodiment of topresent invention the colorants in the two or more color inkjet inks arepreferably present in exactly the same amounts, but small variations inconcentrations when manufacturing the inkjet inks are acceptable. Mostpreferably, the amount of a colorant in a first ink of the two or morecolor inkjet inks differs preferably by no more than 10 wt %, morepreferably by no more than 5 wt % based upon the weight of the colorantin the first ink with the amount of the colorant in a second inkjet inkof the two or more color inkjet inks.

Table 1 gives an illustration of how the magenta and cyan inks couldlook like in a solvent based inkjet ink set. The magenta inks allcontain the same magenta pigment and polymeric dispersant each in anamount of 4.0 wt % based upon the total weight of the ink. The solventmixtures all contain the same solvent 1 in the same amount, which couldbe the consequence of the preparation method of the inkjet inks. Usuallyfirst a concentrated pigment dispersion is prepared using one or moredispersion solvents (here a single dispersion solvent “solvent 1” isused), which is then diluted with one or more ink solvents (heresolvents 2-4) and other compounds such as surfactants, stabilizers andthe like. The ink solvents 2 to 4 are present in differentconcentrations or absent in the three magenta inks A, B and C.Approximately the same mixture of solvents 1 to 4 are used in thecorresponding three cyan inks D, E and F. For example, the cyan ink Dcorresponds with the magenta ink A. On selecting a specific ink-receiverthe magenta inks A, B and C will be mixed together in a certain ratio;the same ratio will normally be used for mixing the corresponding inksD, E and F such that, for example, the adhesion properties of themagenta ink mixture and the cyan ink mixture will be more or less thesame on that specific substrate.

TABLE 1 Compound Magenta inks Cyan inks (wt %) A B C D E F MagentaPigment 4.0 4.0 4.0 — — — Cyan Pigment — — — 3.0 3.0 3.0 Polymeric 4.04.0 4.0 — — — dispersant 1 Polymeric — — — 3.0 3.0 3.0 dispersant 2Solvent 1 25.0 25.0 25.0 27.0 27.0 27.0 Solvent 2 40.0 — — 40.0 — —Solvent 3 25.0 40.0 — 25.0 40.0 — Solvent 4 — 25.0 65.0 — 25.0 65.0Stabilizer 2.0 2.0 2.0 2.0 2.0 2.0

In one embodiment the ink set includes two color inkjet inks of the samecolor and color density wherein a first color inkjet ink includes alarge amount of one or more surfactants and a second color inkjet inkdiffers only by lacking a surfactant or containing a small amount of oneor more surfactants. By mixing the two color inkjet inks in an inksupply system any desired surface tension of the ink mixture can beobtained provided that suitable surfactants were chosen and that thesewere present in high enough amounts in the first color inkjet ink toencompass the whole range of desired surface tensions. Controlling thesurface tension allows the printer to obtain the same spreadingcharacteristics on ink-receivers differing in surface energy.

In a preferred embodiment, both the adhesion and the surface tension iscontrolled by mixing inks in a certain ratio. Table 2 illustrates thisfor an inkjet ink set including four magenta inks. In this case, thesurfactant was added to ink D which corresponds with ink B with theexception of the amount of solvents 3 and 4 for which a correction wasnecessary to compensate for the addition of the surfactant, and also inorder to keep the magenta pigment concentration constant. In a certainink mixture of inks A, B and C optimized for adhesion on a specificsubstrate, an amount of ink B can be replaced by ink D without changingthe adhesion properties, but still adapting the ink mixture to a certainsurface tension for reaching the desired ink spreading characteristicson the substrate. The choice for ink B could be given by the fact thatthe printer noticed that ink B was most frequently used in largeramounts in the ink mixture. Another option could be to include also asmall amount of surfactant in inks A and/or C and use the ink D to“fine-tune” the surface tension of the ink mixture.

TABLE 2 Magenta inks Compound (wt %) A B C D Magenta Pigment 4.0 4.0 4.04.0 Polymeric 4.0 4.0 4.0 4.0 dispersant Solvent 1 22.0 22.0 22.0 22.0Solvent 2 40.0 — — — Solvent 3 30.0 35.0 — 34.0 Solvent 4 — 35.0 70.034.0 Surfactant — — — 2.0

The inkjet printing method according to the present invention could alsobe advantageously used in combination with radiation curable inkjet inksto improve dispersion stability and/or curing speed. This is exemplifiedby Table 3, wherein ink A includes a photo-initiator and apolymerisation inhibitor but lacks the polymerization synergist, whileink B contains this activating synergist but lacks a photo-initiator.Both inks separately may show a very good shelf-life stability, but oncemixed can exhibit a poor stability due to rapidly inducedpolymerization. Mixing these inks just prior to jetting can beadvantageously used to obtain high curing speed.

TABLE 3 Magenta inks Compound (wt %) A B Magenta Pigment 4.0 4.0Polymeric 4.0 4.0 dispersant Monomer 1 22.0 22.0 Monomer 2 40.0 40.0Monomer 3 10.0 10.0 Photo-initiator 10.0 — Polymerization 9.0 —inhibitor Polymerization — 19.0 synergist Surfactant 1.0 1.0

Same Color and Color Density

In the context of this invention the following procedure was used todetermine whether two inks have the same color and density.

Starting from the ink formulation that was used in the printer,dilutions of 1:1000 in mass were prepared. It is evident that thesolvent for the dilution must be compatible with the ink dispersion,i.e. the solvent should be chosen such that the chemical and physicaldispersion stability is maintained, because otherwise color changes dueto additional light scattering for example due to particleagglomerations might occur. Preferably, the dilution solvent is selectedfrom one or more liquid components of the ink. In the examples givenbelow, DPDGA was used as diluting solvent.

From the diluted ink dispersions transmission measurements τ werecollected with a spectrophotometer. Measurements were based on thefollowing geometry: direct illumination and diffusely integratingdetection. An example of such a spectrophotometer is the Double BeamSpectrophotometer Lambda 900 from Perkin Elmer realizing the measurementgeometry τ(8/d) according to ASTM E179-96.

Quartz cuvettes with 10 mm optical path were filled with the dilutedinks and then placed in contact to the entrance port of the integratingsphere. For the reference measurement, the same Quartz cuvette filledwith the neat solvent was used. The transmission spectra of the dilutedinks were divided by the transmission spectra of the referencemeasurement in order to correct for the diluting solvent and the Quartzcuvette. From these spectra the CIE L*a*b* coordinates were calculatedaccording to ASTM E308-01 based on the CIE 1931 standard observer (2deg) and D50 illuminant. From the CIE L*a*b* coordinates the CIE ΔE2000Color Difference was calculated with the industry dependent parametersK_(L), K_(C) and K_(H) set to unity (1).

In view of the application field of inkjet printing for which thisinvention is intended, two inks A and B are regarded as having differentcolor and density if CIE ΔE2000>5.0 is obtained for the given observerand illuminant, i.e. no spectral match is required. If the colordifference CIE ΔE2000 between ink A and ink B were larger than 5.0typically a new characterization of the inkjet printing systems in termsof color management is required, whereas differences smaller than 2.0might be compensated by a new linearization of the printer only. In thatsense, two inks A and B having a pair-wise color difference CIE ΔE2000of smaller than or equal to 2.0 are regarded as having same color anddensity.

For more demanding printing applications, a color difference CIE ΔE2000of smaller than or equal to 1.5 is required for same color and density.In an even more restrictive, calorimetric approach, same color anddensity is realized if the color difference CIE ΔE2000 is smaller thanor equal to 1.0.

References:

-   -   ASTM D2244-02 Standard Practice for Calculation of Color        Tolerances and Color Differences from Instrumentally measured        Color Coordinates    -   ASTM E179-96 (2003) Standard Guide for Selection of Geometric        Conditions for Measurements of Reflection and Transmission        Properties of Materials    -   ASTM E308-01 Standard Practice for Computing the Colors of        Objects by Using the CIE system

Inkjet Printer & Ink Supply Systems

Industrial inkjet printers generally include an ink supply system forsupplying ink to an inkjet print head. Inkjet print heads produce dropseither continuously or on demand. “Continuously” means that a continuousstream of ink drops is created, e.g. by pressurizing the ink supply. “Ondemand” differs from “continuous” in that ink drops are only ejectedfrom a print head by manipulation of a physical process to momentarilyovercome surface tension forces that keep the ink in the print head. Theink is held in a nozzle, forming a meniscus. The ink remains in placeunless some other force overcomes the surface tension forces that areinherent in the liquid. The most common practice is to suddenly raisethe pressure on the ink, ejecting it from the nozzle. One category ofdrop-on-demand inkjet print heads uses the physical phenomenon ofelectrostriction, a change in transducer dimension in response to anapplied electric field. Electrostriction is strongest in piezoelectricmaterials and hence these print heads are referred to as piezoelectricprint heads. The very small dimensional change of piezoelectric materialis harnessed over a large area to generate a volume change that is largeenough to squeeze out a drop of ink from a small chamber. Apiezoelectric print head includes a multitude of small ink chambers,arranged in an array, each having an individual nozzle and a percentageof transformable wall area to create the volume changes required toeject an ink drop from the nozzle, in according with electrostrictionprinciples.

In a preferred embodiment the inkjet printer is a drop-on-demand ink jetprinting system having piezoelectric print heads for delivering dropletsof a selectable color ink mixture to an ink-receiver.

The inkjet ink is supplied to the ink ejecting chambers of a print headby an ink supply system that first conditions the ink in order to obtainsmooth operation of the inkjet print head. Conditioning includes, forexample, degassing of the ink and controlling the back-pressure at thenozzle.

It is known that the presence of air bubbles in the ink chamber of apiezoelectric print head often causes operational failure of the printhead. If air is present in the ink chamber, intended pressure changesresulting from piezoelectric deformation of part of the ink chamberwalls will be absorbed by the air, leaving the ink pressure unaffected.The surface tension force of the ink in the nozzle maintains themeniscus and no drops will be ejected from the ink chamber. At thefrequencies at which piezoelectric transducers in piezoelectric printhead are operated, i.e. in the kHz to MHz range, not only air bubblesbut also dissolved air in the ink can cause operation failure asdescribed above. In the prior art, concepts have been disclosed to avoidair bubbles in the ink chamber by creating an air trap upstream the inkchamber, i.e. prior to the ink entering the ink chamber. Solutions havebeen proposed in EP 714779 A (CANON) and U.S. Pat. No. 4,929,963 (HP) inthe form of air buffers or gas separators that allow air bubbles to riseand evacuate from the ink in an intermediate tank before the ink issupplied to the print head.

A second point of attention in ink supply systems is the pressure at thenozzle, which is critical to a well-tuned and good operating print head.Inkjet print heads operate best at a slightly negative nozzle pressureor back-pressure. In practice this is often achieved by maintaining aheight difference between the free ink surface in a vented ink supplytank and the meniscus in the nozzle. That is, the free ink surface inthe vented supply tank is maintained gravimetrically a couple ofcentimeters below the level of the meniscus in the nozzle. This heightdifference established a hydrostatic pressure difference to control theback-pressure at the nozzle. In reciprocating print head configurationsthe ink supply tank is located off axis, i.e. not scanning, becauseotherwise the lowered position of ink supply tank versus the print headwould interfere with the printing medium transport path. Flexible tubingis used to connect the off axis ink supply tank with the on axis printhead, as disclosed in for example U.S. Pat. No. 4,929,963 (HP). Duringacceleration and deceleration of the print head, pressure waves arecreated in the tubes that may significantly disturb the pressure balanceat the meniscus and may lead to weeping of the nozzle in the case of adecrease in negative pressure, or breaking of the meniscus in the caseof an increase in negative pressure and taking air into the ink channel.Many approaches have been proposed to control the back-pressure inreciprocating print head applications. A back-pressure regulationmechanisms in the form of pressure buffers or dampers mounted togetherwith the print head on the reciprocating carriage are disclosed in EP1120257 A (SEIKO EPSON) and U.S. Pat. No. 6,485,137 (APRION DIGITAL).For accelerations and decelerations of the carriage above 1 G theresponse time of these devices is insufficient. In EP 1142713 A (SEIKOEPSON) a vented subtank is used. The subtank serves as a local inkreservoir near the print head and is being filled intermittently from amain tank located off axis. The solution provides a better control ofthe nozzle back-pressure by maintaining a local hydrostatic pressuredifference between the free ink surface of the vented subtank and themeniscus.

Ink Mixing Device

There are no real limitations for selecting the device to mix the inksas long as they are made from materials compatible with the inks, e.g.solvent-resistant materials when solvent inkjet inks are to be mixed.

The inks can be mixed at various locations of the inkjet printer, forexample, directly at the first connection of the inkjet inks to theinkjet printer, near to the inkjet printheads or even inside theprintheads. The smaller the distance between the location of the inkmixing and the printhead nozzles, the less ink is spilled for adaptingto a new ink-receiver to be printed upon.

In one preferred embodiment, the ink mixing device has a compact designso that it is possible to incorporate it into a carriage including anassembly of print heads that moves back and forth along the fast scandirection.

Preferably, an ink mixing device is selected that does not introduce airbubbles into the ink mixture.

Although not required for obtaining a consistent image quality in colorgamut, preferably an ink mixing device is selected wherein the inks aremixed quite accurately.

For some inkjet inks, such as dye based inks, the ink mixing device maysimply consist of conduits that come together in one conduit, whichmakes a number of sharp turns or V-turns in order to mix the inks.

More complex ink mixing device may include pumps, valves, mixingchambers, etc.

If necessary, the ink mixing may be performed with cooling to preventbuild up of heat. For radiation curable inkjet inks, the ink mixing isperformed as much as possible under light conditions in which actinicradiation has been substantially excluded.

In one embodiment, the two or more color inkjet inks of the same colorand color density are supplied to an inkjet print head via a conduitwherein the ink mixture is prepared in-situ in the conduit. A flowcontroller is adapted to selectably meter ink from the two or more colorinkjet ink sources into the conduit between the source of one colorinkjet ink and the ejecting chambers of the print head. Ink supplysystems according to this embodiment are exemplified by FIG. 1 and FIG.2.

In another embodiment, the ink supply system includes an ink mix chamberwherein the two or more color inkjet inks of the same color and colordensity are first mixed in a controlled amount before delivering thisink mixture to the print head. An ink supply system according to thisembodiment is exemplified by FIG. 3.

The two previous embodiments can also be combined to provide an inksupply system wherein at least three color inkjet inks of the same colorand color density are mixed in a controlled amount partly in an ink mixchamber and partly in-situ in a conduit between the ink mix chamber andthe print head. An ink supply system according to this embodiment isexemplified by FIG. 4.

In another embodiment the mixing in a controlled amount of the two ormore color inkjet inks of the same color and color density occurs insidethe print head. An ink supply system according to this embodiment isexemplified by FIG. 5.

Although possible to locate (part of) the ink mixing system inside theprint head, the ink mixing system is preferably separated from the printhead. This allows connection of the ink supply system to a wide range ofalready commercially available print heads and inkjet printers, andhence does not increase the complexity and development cost of printheads. Moreover, the maintenance is much easier in an ink mixing systemnot located inside the print head when, for example, flocculation of theinks would occur.

It should be clear that for an ink set, a mixing device is preferablypresent for each color for which there are two or more color inkjet inksof the same color and color density in the inkjet ink set.

Computing Device

In a preferred embodiment, the ink supply system is connected to acomputer for controlling the ink mixing process. This may include theopening and closing of valves, the control of the flow by pumps, therotation speed of a stirrer and other mechanical settings, to obtain thedesired ink mixture. However, the computer is preferably also used tostore and recall data of ink mixtures used on specific ink-receivers.This allows for fast adjustment of the inkjet printer to a specificink-receiver which had already been printed upon with the same inkjetink set in the past.

In another embodiment, the computer may be used to produce a testpattern of different ink mixtures on a not previously used ink-receiverwhich after examination of the printed pattern allows the selection ofthe ink mixture exhibiting the desired properties of image quality,adhesion, etc. Employing this method each time a new substrate is usedas ink-receiver results in a digital library of ink mixing data forspecific ink-receivers. This ink mixing data include the ratio of theinkjet inks and its relation to image quality and physical properties.The use of a library, more preferably a digital library, leads toenhanced productivity.

For a number of characteristic properties, it is possible to automatethe evaluation of the test pattern of different ink mixtures byincluding, downstream of the printer, a device capable of measuring orevaluating linewidth, edge straightness, mottle, print density, glossand/or color intensity.

Inkjet Ink-Receivers

The ink-receiver suitable for the inkjet printing method according tothe present invention is not restricted to any specific type and can betransparent, translucent or opaque. The ink-receiver may be colored ormetallized. It can be a temporary substrate, e.g. for transferring animage to another substrate after printing. Applications, such as3D-printing and direct printing on wooden doors, panels and ceramics arealso included in the scope of the invention.

Aqueous inks are generally printed on absorbing ink-receivers. Solventbased inkjet inks and radiation curable inks can also be printed onink-receivers substantially non-absorbing for an aqueous solution. Forexample, standard paper is an absorbing ink-receiver. On the other hand,a resin-coated paper, e.g. polyethylene-coated paper orpolypropylene-coated paper, is usually substantially non-absorbing.

The ink-receiver may include a support with at least one ink-receivinglayer. The ink-receiving layer may consist of just one single layer, oralternatively it may be composed of two, three or more layers. Theink-receiving layer may contain one or more polymeric binders andoptionally fillers. The ink-receiving layer, and an optional auxiliarylayer, such as a backing layer for anti-curl and/or adhesive purposes,may further contain well-known conventional ingredients, such assurfactants serving as coating aids, cross-linking agents, plasticizers,cationic substances acting as mordant, light-stabilizers, pH adjusters,anti-static agents, biocides, lubricants, whitening agents and mattingagents.

The ink-receiving layer and the optional auxiliary layer(s) may becross-linked to a certain degree to provide such desired features aswaterfastness and non-blocking characteristics. The cross-linking isalso useful in providing abrasion resistance and resistance to theformation of fingerprints on the element as a result of handling.

Supports suitable for the ink-receiving layers are also suitableink-receivers for solvent based inkjet inks or radiation curable inksand include polymeric substrates such as cellulose acetate propionate,cellulose acetate butyrate, polyesters such as polyethyleneterephthalate (PET) and polyethylene naphthalate (PEN); orientedpolystyrene (OPS); oriented nylon (ONy); polypropylene (PP), orientedpolypropylene (OPP); polyvinyl chloride (PVC); and various polyamides,polycarbonates, polyimides, polyolefins, poly(vinylacetals), polyethersand polysulfonamides, opaque white polyesters and extrusion blends ofpolyethylene terephthalate and polypropylene. Acrylic resins, phenolresins, glass and metals may also be used as an ink-receiver. Othersuitable ink-receiver materials can be found in Modern Approaches toWettability: Theory and Applications. Edited by SCHRADER, Malcolm E., etal. New York: Plenum Press, 1992. ISBN 0306439859.

The ink-receiver may also incorporate mineral particles as fillers, suchas e.g. PET containing CaCO₃, PET containing TiO₂, amorphous PET (APET)and glycolized PET (PETG).

The ink-receiver may be provided with a self-adhesive backlayer.Examples of self-adhesive PVC ink-receivers include MPI™ vinyls fromAVERY-DENNISON, DIGITAL™ vinyls from METAMARK, MULTI-FIX™ digital whitevinyls from MULTI-FIX and GRAFIPRINT™ vinyls from GRAFITYP.

Polyester film substrates and especially polyethylene terephthalate arepreferred for certain applications, particularly types with excellentdimensional stability. When such a polyester is used as theink-receiver, a subbing layer may be employed to improve the bonding ofthe jetted ink layer to the substrate, if it constitutes together withthe unsubbed substrate a substantially non-absorbing ink-receiver.Useful subbing layers for this purpose are well known in thephotographic art and include, for example, polymers of vinylidenechloride such as vinylidene chloride/acrylonitrile/acrylic acidterpolymers or vinylidene chloride/methyl acrylate/itaconic acidterpolymers. Stabilizers, leveling additives, matting agents, adjustingagents for physical film properties such as waxes, may also be added tothe subbing layer as required.

The ink-receiver may also be made from an inorganic material, such as ametal oxide or a metal (e.g. aluminium and steel).

Other suitable ink-receivers may be selected from the group consistingof cardboard, wood, composite boards, coated plastic, canvas, textile,glasses, plant fibre products, leather, magnetic materials and ceramics.

Inkjet Ink Sets

The inkjet ink set according to the present invention includes two ormore color inkjet inks of the same color and color density but includingdifferent solid and/or liquid components.

In a preferred embodiment the two or more color inkjet inks contain thesame colorants. Preferably the amount of each colorant in a first ink ofthe two or more color inkjet inks differs by no more than 10 wt %, morepreferably no more than 5 wt % and most preferably no more than 2 wt %all based upon the total weight of the colorant in the first ink, withthe amount of the colorant in a second inkjet ink of the two or morecolor inkjet inks. Most preferably the color pigments are present in thesame amount in each of the two or more color inkjet inks.

In one embodiment the two or more color inkjet inks contain a differentamount of surfactant and/or different types of surfactant.

In a preferred embodiment the two or more color inkjet inks are solventbased inkjet inks.

In another preferred embodiment the two or more color inkjet inks areradiation curable inkjet inks.

In a further preferred embodiment of radiation curable inkjet inks, aphoto-initiator is present in at least one of the two or more radiationcurable color inkjet inks and absent in at least one other ink of thetwo or more radiation curable color inkjet inks. In a further preferredembodiment, a co-initiator or polymerization synergist is present in atleast one of the two or more color inkjet inks not including thephoto-initiator. In another further preferred embodiment, an inhibitoris present in at least one of the two or more color inkjet inksincluding the photo-initiator.

In another embodiment, the inkjet ink set may further include two ormore colorless inkjet inks including different solid and/or liquidcomponents. These colorless inkjet inks may be used to reduce or enhancethe gloss of the image, for example, with a topcoat layer. A topcoatlayer can also be applied for enhancing durability or solvent resistanceof the image or for reducing the amount of extractables when printing onpackaging materials of food.

The inkjet ink set in the inkjet printer may include the two or morecolor inkjet inks of the same color and color density in a differentvolume. Sometimes one ink can be consumed much faster than another, forexample, because the other ink is tuned towards good adhesion on lessfrequently used substrates. Given the limited shelf-life, especially forradiation curable inks, the waste of ink and allocated costs thereto canbe reduced.

Inkjet Inks

The inkjet inks in an ink set according to the present invention arepreferably non-aqueous inkjet inks. In a non-aqueous inkjet ink thecomponents are present in a dispersion medium which is a non-aqueousliquid at jetting temperature.

The term “non-aqueous liquid” refers to a liquid carrier which shouldcontain no water. However sometimes a small amount, generally less than5 wt % of water based on the total weight of the ink, can be present.This water was not intentionally added but came into the formulation viaother components as a contamination, such as for example polar organicsolvents. Higher amounts of water than 5 wt % tend to make thenon-aqueous inkjet inks instable, preferably the water content is lessthan 1 wt % based on the total weight dispersion medium and mostpreferably no water at all is present.

The two or more color inkjet inks of the inkjet ink set according to thepresent invention preferably contain a pigment as colorant. If thecolorant is not a self-dispersible pigment, the inkjet inks preferablyalso contain a dispersant, more preferably a polymeric dispersant.

The inkjet inks of an ink set according to the present invention mayfurther also contain at least one surfactant.

The inkjet inks of an ink set according to the present invention maycontain at least one humectant to prevent the clogging of the nozzle,due to its ability to slow down the evaporation rate of ink.

The pigmented color inkjet inks according to the present invention maycontain at least one dispersion synergist. A mixture of dispersionsynergists may be used to further improve dispersion stability

The inkjet inks of an ink set according to the present invention ispreferably an inkjet ink selected from the group consisting of anorganic solvent based, an oil based and a curable inkjet ink. Thecurable inkjet ink is preferably radiation curable.

The viscosity of the inkjet ink is preferably smaller than 100 mPa·s at30° C. and at a shear rate of 100 s⁻¹. The viscosity of the inkjet inkis preferably smaller than 30 mPa·s, more preferably lower than 15mPa·s, and most preferably between 2 and 10 mPa·s at a shear rate of 100s⁻¹ and a jetting temperature between 10 and 70° C.

The curable inkjet ink may contain as dispersion medium monomers,oligomers and/or prepolymers possessing different degrees offunctionality. A mixture including combinations of mono-, di-, tri-and/or higher functionality monomers, oligomers or prepolymers may beused. A catalyst called an initiator for initiating the polymerizationreaction may be included in the curable inkjet ink. The initiator can bea thermal initiator, but is preferably a photo-initiator. Thephoto-initiator requires less energy to activate than the monomers,oligomers and/or prepolymers to form the polymer. The photo-initiatorsuitable for use in the curable pigment dispersion may be a Norrish typeI initiator, a Norrish type II initiator or a photo-acid generator.

The curable inkjet inks of an ink set according to the present inventionmay further also contain at least one inhibitor.

A CMYK inkjet ink set may also be extended with one or more extra inkssuch as red, green, blue and orange to further enlarge the color gamutof the image. The CMYK ink set may also be extended by the combinationof full density and light density inks of both color inks and/or blackinks to improve the image quality by lowered graininess.

Colorants

The two or more color inkjet inks of the inkjet ink set according to thepresent invention contain at least one colorant. Colorants used in theinkjet inks may be pigments, dyes or a combination thereof. Organicand/or inorganic pigments may be used.

The two or more radiation curable inkjet inks or solvent based inkjetinks preferably contain pigments as colorants.

The pigments in the inkjet inks may be black, white, cyan, magenta,yellow, red, orange, violet, blue, green, brown, mixtures thereof, andthe like.

The color pigment may be chosen from those disclosed by HERBST, Willy,et al. Industrial Organic Pigments, Production, Properties,Applications. 3rd edition. Wiley-VCH, 2004. ISBN 3527305769.

Particular preferred pigments are C.I. Pigment Yellow 1, 3, 10, 12, 13,14, 17, 55, 65, 73, 74, 75, 83, 93, 97, 109, 111, 120, 128, 138, 139,150, 151, 154, 155, 180, 185 and 213.

Particular preferred pigments are C.I. Pigment Yellow 120, 151, 154,175, 180, 181 and 194.

The most preferred yellow pigments are C.I. Pigment Yellow 120, 139, 150155 and 213.

Particular preferred pigments are C.I. Pigment Red 17, 22, 23, 41, 48:1,48:2, 49:1, 49:2, 52:1, 57:1, 81:1, 81:3, 88, 112, 122, 144, 146, 149,169, 170, 175, 176, 184, 185, 188, 202, 206, 207, 210, 216, 221, 248,251, 254, 255, 264, 270 and 272. For manufacturing decorative laminates,the most preferred are C.I. Pigment Red 254 and C.I. Pigment Red 266.For other non-aqueous inkjet applications the most preferred pigmentsare C.I. Pigment Red 122 and C.I. Pigment Violet 19.

Particular preferred pigments are C.I. Pigment Violet 1, 2, 19, 23, 32,37 and 39.

Particular preferred pigments are C.I. Pigment Blue 15:1, 15:2, 15:3,15:4, 15:6, 16, 56, 61 and (bridged) aluminium phthalocyanine pigments.

Particular preferred pigments are C.I. Pigment Orange 5, 13, 16, 34, 40,43, 59, 66, 67, 69, 71 and 73.

Particular preferred pigments are C.I. Pigment Green 7 and 36.

Particular preferred pigments are C.I. Pigment Brown 6 and 7.

Suitable pigments include mixed crystals of the above particularpreferred pigments. A commercially available example is CinquasiaMagenta RT-355-D from Ciba Specialty Chemicals.

Carbon black is preferred as a pigment for the black inkjet ink.Suitable black pigment materials include carbon blacks such as PigmentBlack 7 (e.g. Carbon Black MA8® from MITSUBISHI CHEMICAL), REGAL® 400R,MOGUL® L, ELFTEX® 320 from CABOT Co., or Carbon Black FW18, SpecialBlack 250, Special Black 350, Special Black 550, PRINTEX® 25, PRINTEX®35, PRINTEX® 55, PRINTEX® 90, PRINTEX® 150T from DEGUSSA. Additionalexamples of suitable pigments are disclosed in U.S. Pat. No. 5,389,133(XEROX).

It is also possible to make mixtures of pigments in the two or morecolor inkjet inks. For some applications, a neutral black inkjet ink ispreferred and can be obtained, for example, by mixing a black pigmentand a cyan pigment into the ink. The inkjet application may also requireone or more spot colors, for example for packaging inkjet printing ortextile inkjet printing. Silver and gold are often desired colors forinkjet poster printing and point-of-sales displays.

Also non-organic pigments may be present in the two or more color inkjetinks. Particular preferred pigments are C.I. Pigment Metal 1, 2 and 3.Illustrative examples of the inorganic pigments include titanium oxide,barium sulfate, calcium carbonate, zinc oxide, lead sulfate, yellowlead, zinc yellow, red iron oxide (III), cadmium red, ultramarine blue,prussian blue, chromium oxide green, cobalt green, amber, titanium blackand synthetic iron black.

Generally pigments are stabilized in the dispersion medium by dispersingagents, such as polymeric dispersants or surfactants. However, thesurface of the pigments can be modified to obtain so-called“self-dispersible” or “self-dispersing” pigments, i.e. pigments that aredispersible in the dispersion medium without dispersants.

Pigment particles in inkjet ink should be sufficiently small to permitfree flow of the ink through the inkjet-printing device, especially atthe ejecting nozzles. It is also desirable to use small particles formaximum color strength and to slow down sedimentation.

The numeric average pigment particle size is preferably between 0.050and 1 μm, more preferably between 0.070 and 0.300 μm and particularlypreferably between 0.080 and 0.200 μm. Most preferably, the numericaverage pigment particle size is no larger than 0.150 μm. However, theaverage pigment particle size for white inkjet inks including, forexample, a titanium dioxide pigment, is preferably between 0.100 and0.300 μm.

The pigment is preferably used in the pigment dispersion used forpreparing the inkjet inks in an amount of 10 to 40 wt %, preferably 15to 30 wt % based on the total weight of the pigment dispersion. In theinkjet ink the pigment is preferably used in an amount of 0.1 to 20 wt%, preferably 1 to 10 wt % based on the total weight of the inkjet ink.

Dyes suitable for the two or more color inkjet inks in the ink setaccording to the present invention include direct dyes, acidic dyes,basic dyes and reactive dyes.

Suitable direct dyes for the two or more color inkjet inks include:

-   -   C.I. Direct Yellow 1, 4, 8, 11, 12, 24, 26, 27, 28, 33, 39, 44,        50, 58, 85, 86, 100, 110, 120, 132, 142, and 144    -   C.I. Direct Red 1, 2, 4, 9, 11, 134, 17, 20, 23, 24, 28, 31, 33,        37, 39, 44, 47, 48, 51, 62, 63, 75, 79, 80, 81, 83, 89, 90, 94,        95, 99, 220, 224, 227 and 343    -   C.I. Direct Blue 1, 2, 6, 8, 15, 22, 25, 71, 76, 78, 80, 86, 87,        90, 98, 106, 108, 120, 123, 163, 165, 192, 193, 194, 195, 196,        199, 200, 201, 202, 203, 207, 236, and 237    -   C.I. Direct Black 2, 3, 7, 17, 19, 22, 32, 38, 51, 56, 62, 71,        74, 75, 77, 105, 108, 112, 117, 154 and 195

Suitable acidic dyes for the two or more color inkjet inks include:

-   -   C.I. Acid Yellow 2, 3, 7, 17, 19, 23, 25, 20, 38, 42, 49, 59,        61, 72, and 99    -   C.I. Acid Orange 56 and 64    -   C.I. Acid Red 1, 8, 14, 18, 26, 32, 37, 42, 52, 57, 72, 74, 80,        87, 115, 119, 131, 133, 134, 143, 154, 186, 249, 254, and 256    -   C.I. Acid Violet 11, 34, and 75    -   C.I. Acid Blue 1, 7, 9, 29, 87, 126, 138, 171, 175, 183, 234,        236, and 249    -   C.I. Acid Green 9, 12, 19, 27, and 41    -   C.I. Acid Black 1, 2, 7, 24, 26, 48, 52, 58, 60, 94, 107, 109,        110, 119, 131, and 155

Suitable reactive dyes for the two or more color inkjet inks include:

-   -   C.I. Reactive Yellow 1, 2, 3, 14, 15, 17, 37, 42, 76, 95, 168,        and 175    -   C.I. Reactive Red 2, 6, 11, 21, 22, 23, 24, 33, 45, 111, 112,        114, 180, 218, 226, 228, and 235    -   C.I. Reactive Blue 7, 14, 15, 18, 19, 21, 25, 38, 49, 72, 77,        176, 203, 220, 230, and 235    -   C.I. Reactive Orange 5, 12, 13, 35, and 95    -   C.I. Reactive Brown 7, 11, 33, 37, and 46    -   C.I. Reactive Green 8 and 19    -   C.I. Reactive Violet 2, 4, 6, 8, 21, 22, and 25    -   C.I. Reactive Black 5, 8, 31, and 39

Suitable basic dyes for the two or more color inkjet inks include:

-   -   C.I. Basic Yellow 11, 14, 21, and 32    -   C.I. Basic Red 1, 2, 9, 12, and 13    -   C.I. Basic Violet 3, 7, and 14    -   C.I. Basic Blue 3, 9, 24, and 25

If the color inkjet ink contains water, dyes can only manifest the idealcolor in an appropriate range of pH value. Therefore, the inkjet inkpreferably further includes a pH adjuster.

Suitable pH adjusters include NaOH, KOH, NEt₃, NH₃, HCl, HNO₃, H₂SO₄ and(poly)alkanolamines such as triethanolamine and2-amino-2-methyl-1-propaniol. Preferred pH adjusters are NaOH and H₂SO₄.

The dyes are used in the two or more color inkjet inks in an amount of0.1 to 30 wt %, preferably 1 to 20 wt % based on the total weight of theinkjet ink.

In a specific embodiment the colorant is a fluorescent colorant used tointroduce security features. Suitable examples of a fluorescent colorantinclude TINOPAL™ grades such as TINOPAL™ SFD, UVITEX™ grades such asUVITEX™ NFW and UVITEX™ OB, all available from CIBA SPECIALTY CHEMICALS;LEUKOPHOR™ grades from CLARIANT and BLANCOPHOR™ grades such asBLANCOPHOR™ REU and BLANCOPHOR™ BSU from BAYER.

Dispersants

Typical polymeric dispersants are copolymers of two monomers but maycontain three, four, five or even more monomers. The properties ofpolymeric dispersants depend on both the nature of the monomers andtheir distribution in the polymer. Suitable copolymeric dispersants havethe following polymer compositions:

-   -   statistically polymerized monomers (e.g. monomers A and B        polymerized into ABBAABAB);    -   alternating polymerized monomers (e.g. monomers A and B        polymerized into ABABABAB);    -   gradient (tapered) polymerized monomers (e.g. monomers A and B        polymerized into AAABAABBABBB);    -   block copolymers (e.g. monomers A and B polymerized into        AAAAABBBBBB) wherein the block length of each of the blocks (2,        3, 4, 5 or even more) is important for the dispersion capability        of the polymeric dispersant;    -   graft copolymers (graft copolymers consist of a polymeric        backbone with side chains attached to the backbone); and    -   mixed forms of these polymers, e.g. blocky gradient copolymers.

Polymeric dispersants may have different polymer architecture includinglinear, comb/branched, star, dendritic (including dendrimers andhyperbranched polymers). A general review on the architecture ofpolymers is given by ODIAN, George, Principles Of Polymerization, 4thedition, Wiley-Interscience, 2004, p. 1-18.

Comb/branched polymers have side branches of linked monomer moleculesprotruding from various central branch points along the main polymerchain (at least 3 branch points).

Star polymers are branched polymers in which three or more eithersimilar or different linear homopolymers or copolymers are linkedtogether to a single core.

Dendritic polymers include the classes of dendrimers and hyperbranchedpolymers. In dendrimers, with well-defined mono-disperse structures, allbranch points are used (multi-step synthesis), while hyperbranchedpolymers have a plurality of branch points and multifunctional branchesthat lead to further branching with polymer growth (one-steppolymerization process).

Suitable polymeric dispersants may be prepared via addition orcondensation type polymerizations. Polymerization methods include thosedescribed by ODIAN, George, Principles Of Polymerization, 4th edition,Wiley-Interscience, 2004, p. 39-606.

Addition polymerization methods include free radical polymerization(FRP) and controlled polymerization techniques. Suitable controlledradical polymerization methods include:

-   -   RAFT: reversible addition-fragmentation chain transfer;    -   ATRP: atom transfer radical polymerization    -   MADIX: reversible addition-fragmentation chain transfer process,        using a transfer active xanthate;    -   Catalytic chain transfer (e.g. using cobalt complexes);    -   Nitroxide (e.g. TEMPO) mediated polymerizations;

Other suitable controlled polymerization methods include:

-   -   GTP: group transfer polymerization;    -   Living cationic (ring-opening) polymerizations;    -   Anionic co-ordination insertion ring-opening polymerization; and    -   Living anionic (ring-opening) polymerization.

Reversible addition-fragmentation transfer (RAFT): controlledpolymerization occurs via rapid chain transfer between growing polymerradicals and dormant polymer chains. A review article on RAFT synthesisof dispersants with different polymeric geometry is given in QUINN J. F.et al., Facile Synthesis of comb, star, and graft polymers viareversible addition-fragmentation chain transfer (RAFT) polymerization,Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 40,2956-2966, 2002.

Group transfer polymerization (GTP): the method of GTP used forsynthesis of AB block copolymers is disclosed by SPINELLI, Harry J, GTPand its use in water based pigment dispersants and emulsion stabilisers,Proc. of 20th Int. Conf. Org. Coat. Sci. Technol., New Platz, N.Y.,State Univ. N.Y., Inst. Mater. Sci. p. 511-518.

The synthesis of dendritic polymers is described in the literature. Thesynthesis of dendrimers in NEWCOME, G. R., et al. Dendritic Molecules:Concepts, Synthesis, Perspectives. VCH: WEINHEIM, 2001. Hyperbranchingpolymerization is described by BURCHARD, W. Solution properties ofbranched macromolecules. Advances in Polymer Science. 1999, vol. 143,no. II, p. 113-194. Hyperbranched materials can be obtained bypolyfunctional polycondensation as disclosed by FLORY, P. J. Molecularsize distribution in three-dimensional polymers. VI. Branched polymercontaining A-R-Bf-1-type units. Journal of the American ChemicalSociety. 1952, vol. 74, p. 2718-1723.

Living cationic polymerizations is e.g. used for the synthesis ofpolyvinyl ethers as disclosed in WO 2005/012444 (CANON), US 20050197424(CANON) and US 20050176846 (CANON). Anionic co-ordination ring-openingpolymerization is e.g. used for the synthesis of polyesters based onlactones. Living anionic ring-opening polymerization is e.g. used forthe synthesis of polyethylene oxide macromonomers.

Free radical Polymerization (FRP) proceeds via a chain mechanism, whichbasically consists of four different types of reactions involving freeradicals: (1) radical generation from non-radical species (initiation),(2) radical addition to a substituted alkene (propagation), (3) atomtransfer and atom abstraction reactions (chain transfer and terminationby disproportionation), and (4) radical-radical recombination reactions(termination by combination).

Polymeric dispersants having several of the above polymer compositionsare disclosed in U.S. Pat. No. 6,022,908 (HP), U.S. Pat. No. 5,302,197(DU PONT) and U.S. Pat. No. 6,528,557 (XEROX).

Suitable statistical copolymeric dispersants are disclosed in U.S. Pat.No. 5,648,405 (DU PONT), U.S. Pat. No. 6,245,832 (FUJI XEROX), U.S. Pat.No. 6,262,207 (3M), US 20050004262 (KAO) and U.S. Pat. No. 6,852,777(KAO).

Suitable alternating copolymeric dispersants are described in US20030017271 (AKZO NOBEL).

Suitable block copolymeric dispersants have been described in numerouspatents, especially block copolymeric dispersants containing hydrophobicand hydrophilic blocks. For example, U.S. Pat. No. 5,859,113 (DU PONT)AB block copolymers, U.S. Pat. No. 6,413,306 (DU PONT) discloses ABCblock copolymers.

Suitable graft copolymeric dispersants are described in CA 2157361 (DUPONT) (hydrophobic polymeric backbone and hydrophilic side chains);other graft copolymeric dispersants are disclosed in U.S. Pat. No.6,652,634 (LEXMARK) and U.S. Pat. No. 6,521,715 (DU PONT).

Suitable branched copolymeric dispersants are described U.S. Pat. No.6,005,023 (DU PONT), U.S. Pat. No. 6,031,019 (KAO) and U.S. Pat. No.6,127,453 (KODAK).

Suitable dendritic copolymeric dispersants are described in e.g. U.S.Pat. No. 6,518,370 (3M), U.S. Pat. No. 6,258,896 (3M), US 2004102541(LEXMARK), U.S. Pat. No. 6,649,138 (QUANTUM DOT), US 2002256230 (BASF),EP 1351759 A (EFKA ADDITIVES) and EP 1295919 A (KODAK).

Suitable designs of polymeric dispersants for inkjet inks are disclosedin SPINELLI, Harry J., Polymeric Dispersants in Inkjet technology,Advanced Materials, 1998, Vol. 10, no. 15, p. 1215-1218.

The monomers and/or oligomers used to prepare the polymeric dispersantcan be any monomer and/or oligomer found in the Polymer Handbook Vol.1+2, 4th edition, edited by J. BRANDRUP et al., Wiley-Interscience,1999.

Polymers useful as pigment dispersants include naturally occurringpolymers, and specific examples thereof include: proteins, such as glue,gelatine, casein, and albumin; naturally occurring rubbers, such as gumarabic and tragacanth; glucosides such as saponin; alginic acid andalginic acid derivatives, such as propylene glycol alginate; andcellulose derivatives, such as methyl cellulose, carboxymethyl celluloseand ethylhydroxy cellulose; wool and silk, and synthetic polymers.

Suitable examples of monomers for synthesising polymeric dispersantsinclude: acrylic acid, methacrylic acid, maleic acid (or there salts),maleic anhydride, alkyl(meth)acrylates (linear, branched and cycloalkyl)such as methyl(meth)acrylate, n-butyl(meth)acrylate,tert-butyl(meth)acrylate, cyclohexyl(meth)acrylate, and2-ethylhexyl(meth)acrylate; aryl(meth)acrylates such asbenzyl(meth)acrylate, and phenyl(meth)acrylate;hydroxyalkyl(meth)acrylates such as hydroxyethyl(meth)acrylate, andhydroxypropyl(meth)acrylate; (meth)acrylates with other types offunctionalities (e.g. oxiranes, amino, fluoro, polyethylene oxide,phosphate substituted) such as glycidyl (meth)acrylate,dimethylaminoethyl(meth)acrylate, trifluoroethyl acrylate,methoxypolyethyleneglycol (meth)acrylate, and tripropyleneglycol(meth)acrylate phosphate; allyl derivatives such as allyl glycidylether; styrenics such as styrene, 4-methylstyrene, 4-hydroxystyrene,4-acetostyrene, and styrene sulfonic acid; (meth)acrylonitrile;(meth)acrylamides (including N-mono and N,N-disubstituted) such asN-benzyl(meth)acrylamide; maleimides such as N-phenyl maleimide; vinylderivatives such as vinyl alcohol, vinylcaprolactam, vinylpyrrolidone,vinylimidazole, vinylnapthalene, and vinyl halides; vinylethers such asvinylmethyl ether; vinylesters of carboxylic acids such as vinylacetate,vinylbutyrate, and vinyl benzoate. Typical condensation type polymersinclude polyurethanes, polyamides, polycarbonates, polyethers,polyureas, polyimines, polyimides, polyketones, polyester, polysiloxane,phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde,polysulfide, polyacetal or combinations thereof.

Suitable copolymeric dispersants are acrylic acid/acrylonitrilecopolymer, vinyl acetate/acrylic ester copolymer, acrylic acid/acrylicester copolymer, styrene/acrylic acid copolymer, styrene/methacrylicacid copolymer, styrene/methacrylic acid/acrylic ester copolymer,styrene/α-methylstyrene/acrylic acid copolymer,styrene/α-methylstyrene/acrylic acid/acrylic ester copolymer,styrene/maleic acid copolymer, styrene/maleic anhydride copolymer,vinylnaphthalene/acrylic acid copolymer, vinylnapthalene/maleic acidcopolymer, vinyl acetate/ethylene copolymer, vinyl acetate/fattyacid/ethylene copolymer, vinyl acetate/maleic ester copolymer, vinylacetate/crotonic acid copolymer, vinyl acetate/acrylic acid copolymer.

Suitable chemistries of copolymeric dispersants also include:

-   -   Copolymers which are the product of a condensation process of        poly(ethylene imine) with a carboxylic acid terminated polyester        (made by addition polymerization); and    -   Copolymers which are the product of a reaction of a        multifunctional isocyanate with:        -   a compound monosubstituted with a group that is capable of            reacting with an isocyanate, e.g. polyester;        -   a compound containing two groups capable of reacting with an            isocyanate (cross-linker); or        -   a compound with at least one basic ring nitrogen and a group            that is capable of reacting with an isocyanate group.

A detailed list of suitable polymeric dispersants is disclosed by MCCUTCHEON, Functional Materials, North American Edition, Glen Rock, N.J.:Manufacturing Confectioner Publishing Co., 1990, p. 110-129.

Suitable pigment stabilisers are also disclosed in DE 19636382 (BAYER),U.S. Pat. No. 5,720,802 (XEROX), U.S. Pat. No. 5,713,993 (DU PONT), WO(XAAR) and U.S. Pat. No. 5,085,689 (BASF).

One polymeric dispersant or a mixture of two or more polymericdispersants may be present to improve the dispersion stability further.Sometimes surfactants can also be used as pigment dispersants, thus acombination of a polymeric dispersant with a surfactant is alsopossible.

The polymeric dispersant can be non-ionic, anionic or cationic innature; salts of the ionic dispersants can also be used.

The polymeric dispersant has preferably a polymerization degree DPbetween 5 and 1000, more preferably between 10 and 500 and mostpreferably between 10 and 100.

The polymeric dispersant has preferably a number average molecularweight Mn between 500 and 30000, more preferably between 1500 and 10000.

The polymeric dispersant has preferably an average molecular weight Mwsmaller than 100000, more preferably smaller than 50000 and mostpreferably smaller than 30000.

The polymeric dispersant has preferably a polymeric dispersity PDsmaller than 2, more preferably smaller than 1.75 and most preferablysmaller than 1.5.

Commercial examples of polymeric dispersants are the following:

-   -   DISPERBYK™ dispersants available from BYK CHEMIE GMBH;    -   SOLSPERSE™ dispersants available from NOVEON;    -   TEGO™ DISPERS™ dispersants from DEGUSSA;    -   EDAPLAN™ dispersants from MÜNZING CHEMIE;    -   ETHACRYL™ dispersants from LYONDELL;    -   GANEX™ dispersants from ISP;    -   DISPEX™ and EFKA™ dispersants from CIBA SPECIALTY CHEMICALS INC;    -   DISPONER™ dispersants from DEUCHEM; and    -   JONCRYL™ dispersants from JOHNSON POLYMER.

Particularly preferred polymeric dispersants include SOLSPERSE™dispersants from NOVEON, EFKA™ dispersants from CIBA SPECIALTY CHEMICALSINC and DISPERBYK™ dispersants from BYK CHEMIE GMBH.

Particularly preferred dispersants for solvent based pigmenteddispersions are SOLSPERSE™ 32000 and 39000 from NOVEON.

Particularly preferred dispersants for oil based pigmented dispersionsare SOLSPERSE™ 11000, 11200, 13940, 16000, 17000 and 19000 from NOVEON.

Particularly preferred dispersants for UV-curable pigmented dispersionsare SOLSPERSE™ 32000 and 39000 dispersants from NOVEON.

The polymeric dispersant is preferably used in an amount of 2 to 600 wt%, more preferably 5 to 200 wt % based on the weight of the pigment.

Dispersion Synergists

The dispersion synergist usually consists of an anionic part and acationic part. The anionic part of the dispersion synergist exhibiting acertain molecular similarity with the color pigment and the cationicpart of the dispersion synergist consists of one or more protons and/orcations to compensate the charge of the anionic part of the dispersionsynergist.

The synergist is preferably added in a smaller amount than the polymericdispersant(s). The ratio of polymeric dispersant/dispersion synergistdepends upon the pigment and should be determined experimentally.Typically the ratio wt % polymeric dispersant/wt % dispersion synergistis selected between 2:1 to 100:1, preferably between 2:1 and 20:1.

Suitable dispersion synergists that are commercially available includeSOLSPERSE™ 5000 and SOLSPERSE™ 22000 from NOVEON.

A particular preferred pigment for the magenta ink used in an inkjet inkset for manufacturing decorative laminates is a diketopyrrolo-pyrrolepigment. For obtaining excellent dispersion stability and quality,preferably a dispersion synergist was used for a diketopyrrolo-pyrrolepigment as those disclosed in pending European Patent ApplicationEP05111360.

In dispersing C.I. Pigment Blue 15:3, the use of a sulfonatedCu-phthalocyanine dispersion synergist, e.g. SOLSPERSE™ 5000 from NOVEONis preferred. Suitable dispersion synergists for yellow inkjet inksinclude those disclosed in pending European Patent ApplicationEP05111357.

Dispersion Media

In one embodiment the dispersion medium consists of organic solvent(s).Suitable organic solvents include alcohols, ketones, esters, ethers,glycols and polyglycols and derivatives thereof, lactones, N-containingsolvents such as amides. Preferably mixtures of one or more of thesesolvents are used.

Examples of suitable alcohols include methyl alcohol, ethyl alcohol,n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, heptyl alcohol,octyl alcohol, cyclohexyl alcohol, benzyl alcohol, phenylethyl alcohol,phenylpropyl alcohol, furfuryl alcohol, anise alcohol andfluoroalcohols.

Examples of suitable ketones include acetone, methyl ethyl ketone,methyl n-propyl ketone, methyl isopropyl ketone, methyl n-butyl ketone,methyl isobutyl ketone, methyl n-amyl ketone, methyl isoamyl ketone,diethyl ketone, ethyl n-propyl ketone, ethyl isopropyl ketone, ethyln-butyl ketone, ethyl isobutyl ketone, di-n-propyl ketone, diisobutylketone, cyclohexanone, methylcyclohexanone and isophorone,2,4-pentanedione and hexafluoroacetone.

Examples of suitable esters include methyl acetate, ethyl acetate,n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate,hexyl acetate, octyl acetate, benzyl acetate, phenoxyethyl acetate,ethyl phenyl acetate, methyl lactate, ethyl lactate, propyl lactate,butyl lactate; methyl propionate, ethyl propionate, benzyl propionate,ethylene carbonate, propylene carbonate, amyl acetate, ethyl benzoate,butyl benzoate, butyl laurate, isopropyl myristate, isopropyl palmirate,triethyl phosphate, tributyl phosphate, diethyl phthalate, dibutylphthalate, diethyl malonate, dipropyl malonate, diethyl succinate,dibutyl succinate, diethyl glutarate, diethyl adipate, dibutyl adipateand diethyl sebacate.

Examples of suitable ethers include butyl phenyl ether, benzyl ethylether, hexyl ether, diethyl ether, dipropyl ether, tetrahydrofuran anddioxane.

Examples of suitable glycols and polyglycols include ethylene glycol,diethylene glycol, triethylene glycol, propylene glycol, dipropyleneglycol and tripropylene glycol.

Examples of suitable glycol and polyglycol derivatives include etherssuch as alkylene glycol mono alkyl ethers, alkylene glycol dialkylethers, polyalkylene glycol monoalkyl ethers, polyalkylene glycoldialkyl ethers and esters of the preceding glycol ethers such as acetateand propionate esters, in case of dialkyl ethers only one ether function(resulting in mixed ether/ester) or both ether functions can beesterized (resulting in dialkyl ester).

Examples of suitable alkylene glycol mono alkyl ethers include ethyleneglycol mono methyl ether, ethylene glycol mono ethyl ether, ethyleneglycol mono propyl ether, ethylene glycol mono butyl ether, ethyleneglycol mono hexyl ether, ethylene glycol mono 2-ethyl-hexyl ether,ethylene glycol mono phenyl ether, propylene glycol mono methyl ether,propylene glycol mono ethyl ether, propylene glycol mono n-propyl ether,propylene glycol mono n-butyl ether, propylene glycol mono iso-butylether, propylene glycol mono t-butyl ether and propylene glycol monophenyl ether.

Examples of suitable alkylene glycol dialkyl ethers include ethyleneglycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycolmethyl ethyl ether, ethylene glycol dibutyl ether, propylene glycoldimethyl ether, propylene glycol diethyl ether and propylene glycoldibutyl ether.

Examples of suitable polyalkylene glycol mono alkyl ethers includediethylene glycol mono methyl ether, diethylene glycol mono ethyl ether,diethylene glycol mono-n-propyl ether, diethylene glycol mono n-butylether, diethylene glycol mono hexyl ether, triethylene glycol monomethyl ether, triethylene mono ethyl ether, triethylene glycol monobutyl ether, dipropylene mono methyl ether, dipropylene glycol monoethyl ether, dipropylene glycol n-propyl ether, dipropylene glycol monon-butyl ether, dipropylene mono t-butyl ether, tripropylene glycol monomethyl ether, tripropylene glycol mono ethyl ether, tripropylene glycolmono n-propyl ether and tripropylene glycol mono n-butyl ether.

Examples of suitable polyalkylene glycol dialkyl ethers includediethylene glycol dimethyl ether, triethylene glycol dimethyl ether,tetraethylene glycol dimethyl ether, diethylene glycol diethyl ether,triethylene glycol diethyl ether, tetraethylene glycol diethyl ether,diethylene glycol methyl ethyl ether, triethylene glycol methyl ethylether, tetraethylene glycol methyl ethyl ether, diethylene glycoldi-n-propyl ether, diethylene glycol di-iso-propyl ether, dipropyleneglycol dimethyl ether, dipropylene glycol diethyl ether, dipropylene din-propyl ether, dipropylene di t-butyl ether, tripropylene glycoldimethyl ether and tripropylene glycol diethyl ether.

Examples of suitable glycol esters include ethylene glycol monomethylether acetate, ethylene glycol monoethyl ether acetate, ethylene glycolmonopropyl ether acetate, ethylene glycol monobutyl ether acetate,diethylene glycol monoethyl ether acetate, diethylene glycol monobutylether acetate, propylene glycol monomethyl ether acetate, propyleneglycol monoethyl ether acetate, dipropylene glycol monomethyl etheracetate and propylene glycol monomethyl ether propionate.

Preferred solvents for use in pigment dispersions and inkjet inks areone or more polyalkyleneglycol dialkylethers represented by the formula(PAG)

wherein,R₁ and R₂ are each independently selected from an alkyl group having 1to 4 carbon atoms;Y represents an ethylene group and/or a propylene group; whereinn is an integer selected from 4 to 20. Preferably a mixture of two ormore polyalkyleneglycol dialkylethers represented by the formula (PAG).

The alkyl groups R₁ and R₂ of the polyalkyleneglycol dialkylethersaccording to Formula (PAG) preferably represent methyl and/or ethyl.Most preferably the alkyl groups R₁ and R₂ are both methyl groups.

In a preferred embodiment the polyalkyleneglycol dialkylethers accordingto Formula (PAG) are polyethylene glycol dialkylethers.

In another preferred embodiment, a mixture of 2, 3, 4 or morepolyalkyleneglycol dialkylethers, more preferably polyethylene glycoldialkylethers are present in the pigment dispersion or inkjet ink.

Suitable mixtures of polyalkyleneglycol dialkylethers for the pigmentdispersions include mixtures of polyethylene glycol dimethyl ethershaving a molecular weight of at least 200, such as Polyglycol DME 200™,Polyglycol DME 250™ and Polyglycol DME 500™ from CLARIANT. Thepolyalkyleneglycol dialkylethers used in non-aqueous inkjet inks havepreferably an average molecular weight between 200 and 800, and morepreferably no polyalkyleneglycol dialkylethers with a molecular weightof more than 800 are present. The mixture of polyalkyleneglycoldialkylethers is preferably a homogeneous liquid mixture at roomtemperature.

Suitable commercial glycol ether solvents include CELLOSOLVE™ solventsand CARBITOL™ solvents from UNION CARBIDE, EKTASOLVE™ solvents fromEASTMAN, DOWANOL™ solvents from DOW, OXITOLL™ solvents, DIOXITOLL™solvents, PROXITOLL™ solvents and DIPROXITOLL™ solvents from SHELLCHEMICAL and ARCOSOLV™ solvents from LYONDELL.

Lactones are compounds having a ring structure formed by ester bonds andcan be of the γ-lactone (5-membered ring structure), δ-lactone(6-membered ring structure) or ε-lactone (7-membered ring structure)types. Suitable examples of lactones include γ-butyrolactone,γ-valerolactone, γ-hexylactone, γ-heptalactone, γ-octalactone,γ-nonalactone, γ-decalactone, γ-undecalactone, δ-valerolactone,δ-hexylactone, δ-heptalactone, δ-octalactone, δ-nonalactone,δ-decalactone, δ-undecalactone and ε-caprolactone.

Suitable examples of N-containing organic solvents include2-pyrrolidone, N-methylpyrrolidone, N-ethyl-2-pyrrolidone,N-octyl-2-pyrrolidone, N-dodecyl-2-pyrrolidone, N,N-dimethylacetamid,N,N-dimethylformamid, acetonitril and N,N-dimethyldodecanamide.

In another embodiment the dispersion medium includes oil types ofliquids, alone or in combination with organic solvent(s). Suitableorganic solvents include alcohols, ketones, esters, ethers, glycols andpolyglycols and derivatives thereof, lactones, N-containing solventssuch as amides, higher fatty acid ester and mixtures of one or more ofthe solvents as described above for solvent based dispersion media.

The amount of polar solvent is preferably lower than the amount of oil.The organic solvent has preferably a high boiling point, preferablyabove 200° C. Examples of suitable combinations are disclosed by EP0808347 A (XAAR) especially for the use of oleyl alcohol and EP 1157070A (MARCONI DATA SYSTEMS) for the combination of oil and volatile organicsolvent.

Suitable oils include saturated hydrocarbons and unsaturatedhydrocarbons, aromatic oils, paraffinic oils, extracted paraffinic oils,napthenic oils, extracted napthenic oils, hydrotreated light or heavyoils, vegetable oils, white oils, petroleum naphtha oils,halogen-substituted hydrocarbons, silicones and derivatives and mixturesthereof.

Hydrocarbons may be selected from straight chain or branched chainaliphatic hydrocarbons, alicyclic hydrocarbons and aromatichydrocarbons. Examples of hydrocarbons are saturated hydrocarbons suchas n-hexane, isohexane, n-nonane, isononane, dodecane and isododecane;unsaturated hydrocarbons such as 1-hexene, 1-heptene and 1-octene;cyclic saturated hydrocarbons such as cyclohexane, cycloheptane,cyclooctane, cyclodecane and decalin; cyclic unsaturated hydrocarbonssuch as cyclohexene, cycloheptene, cyclooctene,1,3,5,7-cyclooctatetraene; and cyclododecene; and aromatic hydrocarbonssuch as benzene, toluene, xylene, naphthalene, phenanthrene, anthraceneand derivatives thereof. In literature the term paraffinic oil is oftenused. Suitable Paraffinic oils can be normal paraffin type (octane andhigher alkanes), isoparaffins (isooctane and higher iso-alkanes) andcycloparaffins (cyclooctane and higher cycloalkanes) and mixtures ofparaffin oils. The term “liquid paraffin” is often used to refer to amixture of mainly including three components of a normal paraffin, anisoparaffin and a monocyclic paraffin, which is obtained by highlyrefining a relatively volatile lubricating oil fraction through asulphuric-acid washing or the like, as described in U.S. Pat. No.6,730,153 (SAKATA INX). Suitable hydrocarbons are also described asde-aromatized petroleum distillates.

Suitable examples of halogenated hydrocarbons include methylenedichloride, chloroform, tetrachloromethane and methyl chloroform. Othersuitable examples of halogen-substituted hydrocarbons includeperfluoro-alkanes, fluorine-based inert liquids and fluorocarboniodides.

Suitable examples of silicone oils include dialkyl polysiloxane (e.g.,hexamethyl disiloxane, tetramethyl disiloxane, octamethyl trisiloxane,hexamethyl trisiloxane, heptamethyl trisiloxane, decamethyltetrasiloxane, trifluoropropyl heptamethyl trisiloxane, diethyltetramethyl disiloxane), cyclic dialkyl polysiloxane (e.g., hexamethylcyclotrisiloxane, octamethyl cyclotetrasiloxane, tetramethylcyclotetrasiloxane, tetra(trifluoropropyl)tetramethylcyclotetrasiloxane), and methylphenyl silicone oil.

White oil is a term used for white mineral oils, which are highlyrefined mineral oils that consist of saturated aliphatic and alicyclicnon-polar hydrocarbons. White oils are hydrophobic, colorless,tasteless, odourless, and do not change color over time.

Vegetable oils include semi-drying oils such as soybean oil, cotton seedoil, sunflower oil, rape seed oil, mustard oil, sesame oil and corn oil;non-drying oils such as olive oil, peanut oil and tsubaki oil; anddrying oils such as linseed oil and safflower oil, wherein thesevegetable oils can be used alone or as a mixture thereof.

Examples of other suitable oils include petroleum oils, non-drying oilsand semi-drying oils.

Commercially available suitable oils include the aliphatic hydrocarbonstypes such as the ISOPAR™ range (isoparaffins) and Varsol/Naphtha rangefrom EXXON CHEMICAL, the SOLTROL™ range and hydrocarbons from CHEVRONPHILLIPS CHEMICAL, and the SHELLSOL™ range from SHELL CHEMICALS.

Suitable commercial normal paraffins include the NORPAR™ range fromEXXON MOBIL CHEMICAL.

Suitable commercial napthenic hydrocarbons include the NAPPAR™ rangefrom EXXON MOBIL CHEMICAL.

Suitable commercial de-aromatized petroleum distillates include theEXXSOL™ D types from EXXON MOBIL CHEMICAL.

Suitable commercial fluoro-substituted hydrocarbons includefluorocarbons from DAIKIN INDUSTRIES LTD, Chemical Division.

Suitable commercial silicone oils include the silicone fluid ranges fromSHIN-ETSU CHEMICAL, Silicone Division.

Suitable commercial white oils include WITCO™ white oils from CROMPTONCORPORATION.

If the non-aqueous pigment dispersion is a curable pigment dispersion,the dispersion medium includes one or more monomers and/or oligomers toobtain a liquid dispersion medium. Sometimes, it can be advantageous toadd a small amount of an organic solvent to improve the dissolution ofthe dispersant. The content of organic solvent should be lower than 20wt % based on the total weight of the inkjet ink. In other cases, it canbe advantageous to add a small amount of water, for example, to improvethe spreading of the inkjet ink on a hydrophilic surface, but preferablythe inkjet ink contains no water.

Preferred organic solvents include alcohols, aromatic hydrocarbons,ketones, esters, aliphatic hydrocarbons, higher fatty acids, carbitols,cellosolves, higher fatty acid esters. Suitable alcohols includemethanol, ethanol, propanol and 1-butanol, 1-pentanol, 2-butanol,t.-butanol. Suitable aromatic hydrocarbons include toluene, and xylene.Suitable ketones include methyl ethyl ketone, methyl isobutyl ketone,2,4-pentanedione and hexafluoroacetone. Also glycol, glycolethers,N-methylpyrrolidone, N,N-dimethylacetamid, N,N-dimethylformamid may beused.

In the case of a curable inkjet ink, the dispersion medium preferablyconsists of monomers and/or oligomers.

Monomers and Oligomers

Any monomer or oligomer may be used as curable compound for the curableinkjet ink. A combination of monomers, oligomers and/or prepolymers mayalso be used. The monomers, oligomers and/or prepolymers may possessdifferent degrees of functionality, and a mixture including combinationsof mono-, di-, tri- and higher functionality monomers, oligomers and/orprepolymers may be used. The viscosity of the inkjet ink can be adjustedby varying the ratio between the monomers and oligomers.

Any method of conventional radical polymerization, photo-curing systemusing photo acid or photo base generator, or photo induction alternatingcopolymerization may be employed. In general, radical polymerization andcationic polymerization are preferred, and photo induction alternatingcopolymerization needing no initiator may also be employed. Furthermore,a hybrid system of combinations of these systems is also effective.

Cationic polymerization is superior in effectiveness due to lack ofinhibition of the polymerization by oxygen, however it is expensive andslow, especially under conditions of high relative humidity. If cationicpolymerization is used, it is preferred to use an epoxy compoundtogether with an oxetane compound to increase the rate ofpolymerization. Radical polymerization is the preferred polymerizationprocess.

Any polymerizable compound commonly known in the art may be employed.Particularly preferred for use as a radiation curable compound in theradiation curable inkjet ink are monofunctional and/or polyfunctionalacrylate monomers, oligomers or prepolymers, such as isoamyl acrylate,stearyl acrylate, lauryl acrylate, octyl acrylate, decyl acrylate,isoamylstyl acrylate, isostearyl acrylate, 2-ethylhexyl-diglycolacrylate, 2-hydroxybutyl acrylate, 2-acryloyloxyethylhexahydrophthalicacid, butoxyethyl acrylate, ethoxydiethylene glycol acrylate,methoxydiethylene glycol acrylate, methoxypolyethylene glycol acrylate,methoxypropylene glycol acrylate, phenoxyethyl acrylate,tetrahydrofurfuryl acrylate, isobornyl acrylate, 2-hydroxyethylacrylate, 2-hydroxypropyl acrylate, 2-hydroxy-3-phenoxypropyl acrylate,vinyl ether acrylate, vinyl ether ethoxy(meth)acrylate,2-acryloyloxyethylsuccinic acid, 2-acryloyxyethylphthalic acid,2-acryloxyethyl-2-hydroxyethyl-phthalic acid, lactone modified flexibleacrylate, and t-butylcyclohexyl acrylate, triethylene glycol diacrylate,tetraethylene glycol diacrylate, polyethylene glycol diacrylate,dipropylene glycol diacrylate, tripropylene glycol diacrylate,polypropylene glycol diacrylate, 1,4-butanediol diacrylate,1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, neopentyl glycoldiacrylate, dimethylol-tricyclodecane diacrylate, bisphenol A EO(ethylene oxide) adduct diacrylate, bisphenol A PO (propylene oxide)adduct diacrylate, hydroxypivalate neopentyl glycol diacrylate,propoxylated neopentyl glycol diacrylate, alkoxylateddimethyloltricyclodecane diacrylate and polytetramethylene glycoldiacrylate, trimethylolpropane triacrylate, EO modifiedtrimethylolpropane triacrylate, tri(propylene glycol) triacrylate,caprolactone modified trimethylolpropane triacrylate, pentaerythritoltriacrylate, pentaerithritol tetraacrylate, pentaerythritolethoxytetraacrylate, dipentaerythritol hexaacrylate, ditrimethylolpropanetetraacrylate, glycerinpropoxy triacrylate, and caprolactam modifieddipentaerythritol hexaacrylate, or an N-vinylamide such as,N-vinylcaprolactam or N-vinylformamide; or acrylamide or a substitutedacrylamide, such as acryloylmorpholine.

Other suitable monofunctional acrylates include caprolactone acrylate,cyclic trimethylolpropane formal acrylate, ethoxylated nonyl phenolacrylate, isodecyl acrylate, isooctyl acrylate, octyldecyl acrylate,alkoxylated phenol acrylate, tridecyl acrylate and alkoxylatedcyclohexanone dimethanol diacrylate.

Other suitable difunctional acrylates include alkoxylated cyclohexanonedimethanol diacrylate, alkoxylated hexanediol diacrylate, dioxane glycoldiacrylate, dioxane glycol diacrylate, cyclohexanone dimethanoldiacrylate, diethylene glycol diacrylate and neopentyl glycoldiacrylate.

Other suitable trifunctional acrylates include propoxylated glycerinetriacrylate and propoxylated trimethylolpropane triacrylate.

Other higher functional acrylates include di-trimethylolpropanetetraacrylate, dipentaerythritol pentaacrylate, ethoxylatedpentaerythritol tetraacrylate, methoxylated glycol acrylates andacrylate esters.

Furthermore, methacrylates corresponding to the above-mentionedacrylates may be used with these acrylates. Of the methacrylates,methoxypolyethylene glycol methacrylate, methoxytriethylene glycolmethacrylate, hydroxyethyl methacrylate, phenoxyethyl methacrylate,cyclohexyl methacrylate, tetraethylene glycol dimethacrylate, andpolyethylene glycol dimethacrylate are preferred due to their relativelyhigh sensitivity and higher adhesion to an ink-receiver surface.

Furthermore, the inkjet inks may also contain polymerizable oligomers.Examples of these polymerizable oligomers include epoxy acrylates,aliphatic urethane acrylates, aromatic urethane acrylates, polyesteracrylates, and straight-chained acrylic oligomers.

Suitable examples of styrene compounds are styrene, p-methylstyrene,p-methoxystyrene, β-methylstyrene, p-methyl-β-methylstyrene,α-methylstyrene and p-methoxy-β-methylstyrene.

Suitable examples of vinylnaphthalene compounds are 1-vinylnaphthalene,α-methyl-1-vinylnaphthalene, β-methyl-1-vinylnaphthalene,4-methyl-1-vinylnaphthalene and 4-methoxy-1-vinylnaphthalene.

Suitable examples of N-vinyl compounds are N-vinylcarbazole,N-vinylpyrrolidone, N-vinylindole, N-vinylpyrrole, N-vinylphenothiazine,N-vinylacetoanilide, N-vinylethylacetoamide, N-vinylsuccinimide,N-vinylphthalimide, N-vinylcaprolactam and N-vinylimidazole.

The cationically polymerizable compound of the inkjet ink can be one ormore monomers, one or more oligomers or a combination thereof.

Suitable examples of cationically curable compounds can be found inAdvances in Polymer Science, 62, pages 1 to 47 (1984) by J. V. Crivello.

The cationic curable compound may contain at least one olefin,thioether, acetal, thioxane, thietane, aziridine, N-, O-, S- orP-heterocycle, aldehyde, lactam or cyclic ester group.

Examples of cationic polymerizable compounds include monomers and/oroligomers epoxides, vinyl ethers, styrenes, oxetanes, oxazolines,vinylnaphthalenes, N-vinyl heterocyclic compounds, tetrahydrofurfurylcompounds.

The cationically polymerizable monomer can be mono-, di- ormulti-functional or a mixture thereof.

Suitable cationic curable compounds having at least one epoxy group arelisted in the “Handbook of Epoxy Resins” by Lee and Neville, McGraw HillBook Company, New York (1967) and in “Epoxy Resin Technology” by P. F.Bruins, John Wiley and Sons New York (1968).

Examples of cationic curable compounds having at least one epoxy groupinclude 1,4-butanediol diglycidyl ether,3-(bis(glycidyloxymethyl)methoxy)-1,2-propane diol, limonene oxide,2-biphenyl glycidyl ether,3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate,epichlorohydrin-bisphenol S based epoxides, epoxidized styrenics andmore epichlorohydrin-bisphenol F and A based epoxides and epoxidizednovolaks.

Suitable epoxy compounds including at least two epoxy groups in themolecule are alicyclic polyepoxide, polyglycidyl ester of polybasicacid, polyglycidyl ether of polyol, polyglycidyl ether ofpolyoxyalkylene glycol, polyglycidyl ester of aromatic polyol,polyglycidyl ether of aromatic polyol, urethane polyepoxy compound, andpolyepoxy polybutadiene.

Examples of cycloaliphatic bisepoxides include copolymers of epoxidesand hydroxyl components such as glycols, polyols, or vinyl ether, suchas 3,4-epoxycyclohexylmethyl-3′, 4′-epoxycyclohexylcarboxylate;bis(3,4-epoxycylohexylmethyl) adipate; limonene bisepoxide; diglycidylester of hexahydrophthalic acid.

Examples of vinyl ethers having at least one vinyl ether group includeethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecylvinyl ether, cyclohexyl vinyl ether, butanediol divinyl ether, hydroxylbutyl vinyl ether, cyclohexane dimethanol monovinyl ether, phenyl vinylether, p-methylphenyl vinyl ether, p-methoxyphenyl vinyl ether,α-methylphenyl vinyl ether, β-methylisobutyl vinyl ether andβ-chloroisobutyl vinyl ether, diethyleneglycol divinyl ether,triethylene glycol divinyl ether, n-propyl vinyl ether, isopropyl vinylether, dodecyl vinyl ether, diethylene glycol monovinyl ether,cyclohexanedimethanol divinyl ether, 4-(vinyloxy)butyl benzoate,bis[4-(vinyl oxy)butyl]adipate, bis[4-(vinyl oxy)butyl]succinate,4-(vinyloxy methyl)cyclohexylmethyl benzoate,bis[4-(vinyloxy)butyl]isophthalate,bis[4-(vinyloxymethyl)cyclohexylmethyl]glutarate,tris[4-(vinyloxy)butyl]trimellitate, 4-(vinyloxy)butyl steatite,bis[4-(vinyloxy)butyl]hexanediylbiscarbamate,bis[4-(vinyloxy)methyl]cyclohexyl]methyl]terephthalate,bis[4-(vinyloxy)methyl]cyclohexyl]methyl]isophthalate,bis[4-(vinyloxy)butyl](4-methyl-1,3-phenylene)-biscarbamate,bis[4-vinyloxy)butyl](methylenedi-4,1-phenylene)biscarbamate and3-amino-1-propanol vinyl ether.

Suitable examples of oxetane compounds having at least one oxetane groupinclude 3-ethyl-3-hydroloxymethyl-1-oxetane, the oligomeric mixture1,4-bis[3-ethyl-3-oxetanyl methoxy)methyl]benzene,3-ethyl-3-phenoxymethyl-oxetane, bis ([1-ethyl(3-oxetanil)]methyl)ether,3-ethyl-3-[(2-ethylhexyloxy)methyl]oxetane, 3-ethyl-[(tri-ethoxysilylpropoxy)methyl]oxetane and 3,3-dimethyl-2(p-methoxy-phenyl)-oxetane.

A preferred class of monomers and oligomers which can be used in bothradiation and cationically curable compositions are vinyl etheracrylates such as those described in U.S. Pat. No. 6,310,115 (AGFA),incorporated herein by reference. Particularly preferred compounds are2-(2-vinyloxyethoxy)ethyl(meth)acrylate, most preferably the compound is2-(2-vinyloxyethoxy)ethyl acrylate.

Initiators

The curable inkjet ink preferably also contains an initiator. Theinitiator typically initiates the polymerization reaction. The initiatorcan be a thermal initiator, but is preferably a photo-initiator. Thephoto-initiator requires less energy to activate than the monomers,oligomers and/or prepolymers to form the polymer. The photo-initiatorsuitable for use in the curable inkjet inks may be a Norrish type Iinitiator, a Norrish type II initiator or a photo-acid generator.

Thermal initiator(s) suitable for use in the curable inkjet ink includetert-amyl peroxybenzoate, 4,4-azobis(4-cyanovaleric acid),1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azobisisobutyronitrile(AIBN), benzoyl peroxide, 2,2-bis(tert-butylperoxy)butane,1,1-bis(tert-butylperoxy)cyclohexane,1,1-bis(tert-butylperoxy)cyclohexane,2,5-bis(tert-butylperoxy)-2,5-dimethylhexane,2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne,bis(1-(tert-butylperoxy)-1-methylethyl)benzene,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylhydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butylperoxybenzoate, tert-butylperoxy isopropyl carbonate, cumenehydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroylperoxide, 2,4-pentanedione peroxide, peracetic acid and potassiumpersulfate.

The photo-initiator or photo-initiator system absorbs light and isresponsible for the production of initiating species, such as freeradicals and cations. Free radicals and cations are high-energy speciesthat induce polymerization of monomers, oligomers and polymers and withpolyfunctional monomers and oligomers thereby also inducingcross-linking.

Irradiation with actinic radiation may be realized in two steps bychanging wavelength or intensity. In such cases it is preferred to use 2types of photo-initiator together.

A combination of different types of initiator, for example, aphoto-initiator and a thermal initiator can also be used.

A preferred Norrish type I-initiator is selected from the groupconsisting of benzoinethers, benzil ketals, α,α-dialkoxyacetophenones,α-hydroxyalkylphenones, α-aminoalkylphenones, acylphosphine oxides,acylphosphine sulphides, α-haloketones, α-halosulfones andα-halophenylglyoxalates.

A preferred Norrish type II-initiator is selected from the groupconsisting of benzophenones, thioxanthones, 1,2-diketones andanthraquinones. A preferred co-initiator is selected from the groupconsisting of an aliphatic amine, an aromatic amine and a thiol.Tertiary amines, heterocyclic thiols and 4-dialkylamino-benzoic acid areparticularly preferred as co-initiator.

Suitable photo-initiators are disclosed in CRIVELLO, J. V., et al.VOLUME III: Photoinitiators for Free Radical Cationic. 2nd edition.Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998. p.287-294.

Specific examples of photo-initiators may include, but are not limitedto, the following compounds or combinations thereof: benzophenone andsubstituted benzophenones, 1-hydroxycyclohexyl phenyl ketone,thioxanthones such as isopropylthioxanthone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzildimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphineoxide, 2,4,6trimethylbenzoyldiphenylphosphine oxide,2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one,2,2-dimethoxy-1,2-diphenylethan-1-one or 5,7-diiodo-3-butoxy-6-fluorone,diphenyliodonium fluoride and triphenylsulfonium hexafluophosphate.

Suitable commercial photo-initiators include IRGACURE™ 184, IRGACURE™500, IRGACURE™ 907, IRGACURE™ 369, IRGACURE™ 1700, IRGACURE™ 651,IRGACURE™ 819, IRGACURE™ 1000, IRGACURE™ 1300, IRGACURE™ 1870, DAROCUR™1173, DAROCUR™ 2959, DAROCUR™ 4265 and DAROCUR™ ITX available from CIBASPECIALTY CHEMICALS, Lucerin TPO available from BASF AG, ESACURE™ KT046,ESACURE™ KIP150, ESACURE™ KT37 and ESACURE™ EDB available from LAMBERTI,H-Nu™ 470 and H-Nu™ 470X available from SPECTRA GROUP Ltd.

Suitable cationic photo-initiators include compounds, which form aproticacids or Bronstead acids upon exposure to ultraviolet and/or visiblelight sufficient to initiate polymerization. The photo-initiator usedmay be a single compound, a mixture of two or more active compounds, ora combination of two or more different compounds, i.e. co-initiators.Non-limiting examples of suitable cationic photo-initiators arearyldiazonium salts, diaryliodonium salts, triarylsulphonium salts,triarylselenonium salts and the like.

The curable inkjet ink may contain a photo-initiator system containingone or more photo-initiators and one or more sensitizers that transferenergy to the photo-initiator(s). Suitable sensitizers includephotoreducible xanthene, fluorene, benzoxanthene, benzothioxanthene,thiazine, oxazine, coumarin, pyronine, porphyrin, acridine, azo, diazo,cyanine, merocyanine, diarylmethyl, triarylmethyl, anthraquinone,phenylenediamine, benzimidazole, fluorochrome, quinoline, tetrazole,naphthol, benzidine, rhodamine, indigo and/or indanthrene dyes. Theamount of the sensitizer is in general from 0.01 to 15 wt %, preferablyfrom 0.05 to 5 wt %, based in each case on the total weight of thecurable inkjet ink.

In order to increase the photosensitivity further, the curable inkjetink may additionally contain co-initiators. For example, the combinationof titanocenes and trichloromethyl-s-triazines, of titanocenes andketoxime ethers and of acridines and trichloromethyl-s-triazines isknown. A further increase in sensitivity can be achieved by addingdibenzalacetone or amino acid derivatives. The amount of co-initiator orco-initiators is in general from 0.01 to 20 wt %, preferably from 0.05to 10 wt %, based in each case on the total weight of the curable inkjetink.

Suitable examples of co-initiators can be categorized in 4 groups:

(1) tertiary aliphatic amines such as methyldiethanolamine,dimethylethanolamine, triethanolamine, triethylamine andN-methylmorpholine;(2) aromatic amines such as amylparadimethylaminobenzoate,2-n-butoxyethyl-4-(dimethylamino)benzoate,2-(dimethylamino)ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and2-ethylhexyl-4-(dimethylamino)benzoate;(3) (meth)acrylated amines such as dialkylamino alkyl(meth)acrylates(e.g., diethylaminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates(e.g., N-morpholinoethyl-acrylate); and(4) amides or ureas.The preferred co-initiators are aminobenzoates.

A preferred initiator system is2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetraphenyl-(7CI,8CI)4,4′-Bi-4H-imidazole corresponding to the chemical formula:

in the presence of a co-initiator such as 2-mercapto benzoxazole.

Another preferred type of initiator is an oxime ester. A suitableexample has as chemical formula:

A preferred amount of initiator is 0.3-50 wt % of the total weight ofthe curable liquid, and more preferably 1-15 wt % of the total weight ofthe curable inkjet ink.

Irradiation with actinic radiation may be realized in two steps bychanging wavelength or intensity. In such cases it is preferred to use 2types of photo-initiator together.

Inhibitors

Suitable polymerization inhibitors include phenothiazine, phenol typeantioxidants, hindered amine light stabilizers, phosphor typeantioxidants, hydroquinone monomethyl ether commonly used in(meth)acrylate monomers, and hydroquinone, t-butylcatechol, pyrogallolmay also be used. Of these, a phenol compound having a double bond inmolecules derived from acrylic acid is particularly preferred due to itshaving a polymerization-restraining effect even when heated in a closed,oxygen-free environment. Suitable inhibitors are, for example,SUMILIZER™ GA-80, SUMILIZER™ GM and SUMILIZER™ GS produced by SumitomoChemical Co., Ltd, Ciba IRGASTAB™ UV10 from CIBA Specialty Products andGENORAD™ 16 available from RAHN.

Since excessive addition of these polymerization inhibitors will lowerthe sensitivity to curing, it is preferred that the amount capable ofpreventing polymerization be determined prior to blending. The amount ofa polymerization inhibitor is generally between 200 and 20,000 ppm ofthe total weight of the curable inkjet ink.

Surfactants

The surfactant(s) can be anionic, cationic, non-ionic, or zwitter-ionicand are usually added in a total quantity less than 20 wt % based on thetotal weight of the inkjet ink and particularly in a total less than 10wt % based on the total weight of the inkjet ink.

Suitable surfactants include fluorinated surfactants, fatty acid salts,ester salts of a higher alcohol, alkylbenzene sulphonate salts,sulphosuccinate ester salts and phosphate ester salts of a higheralcohol (for example, sodium dodecylbenzenesulphonate and sodiumdioctylsulphosuccinate), ethylene oxide adducts of a higher alcohol,ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of apolyhydric alcohol fatty acid ester, and acetylene glycol and ethyleneoxide adducts thereof (for example, polyoxyethylene nonylphenyl ether,and SURFYNOL™ 104, 104H, 440, 465 and TG available from AIR PRODUCTS &CHEMICALS INC.).

For non-aqueous inkjet inks preferred surfactants are selected fromfluoro surfactants (such as fluorinated hydrocarbons) and siliconesurfactants. The silicones are typically siloxanes and can bealkoxylated, polyether modified, polyether modified hydroxy functional,amine modified, epoxy modified and other modifications or combinationsthereof. Preferred siloxanes are polymeric, for examplepolydimethylsiloxanes.

In a curable inkjet ink a fluorinated or silicone compound may be usedas a surfactant, preferably a cross-linkable surfactant is used.Polymerizable monomers having surface-active effects include siliconemodified acrylates, silicone modified methacrylates, acrylatedsiloxanes, polyether modified acrylic modified siloxanes, fluorinatedacrylates, and fluorinated methacrylates. Polymerizable monomers havingsurface-active effects can be mono-, di-, tri- or higher functional(meth)acrylates or mixtures thereof.

Binders

The two or more color inkjet inks of the inkjet ink set according to thepresent invention may include a binder resin. The binder functions as aviscosity controlling agent and also provides fixability relative to asubstrate, e.g. a polyvinyl chloride substrate. The binder preferablyhas a good solubility in the solvent(s).

Non-aqueous inkjet ink compositions preferably include a binder resin.The binder functions as a viscosity controlling agent and also providesfixability relative to the polymeric resin substrate, e.g. a polyvinylchloride substrate, also called vinyl substrate. The binder must beselected to have a good solubility in the solvent(s).

Suitable examples of binder resins include acrylic resins, modifiedacrylic resins, styrene acrylic resins, acrylic copolymers, acrylateresins, aldehyde resins, rosins, rosin esters, modified rosins andmodified rosin resins, acetyl polymers, acetal resins such as polyvinylbutyral, ketone resins, phenolic resins and modified phenolic resins,maleic resins and modified maleic resins, terpene resins, polyesterresins, polyamide resins, polyurethane resins, epoxy resins, vinylresins, vinyl chloride-vinyl acetate copolymer resins, cellulose typeresins such as nitro cellulose, cellulose acetopropionate and celluloseacetate butyrate, and vinyl toluene-α-methylstylene copolymer resin.These binders may be used alone or in a mixture thereof. The binder ispreferably a film-forming thermoplastic resin.

The amount of binder resin in inkjet ink is preferably in the range of0.1 to 30 wt %, more preferably 1 to 20 wt %, most preferably 2 to 10 wt% based on the total weight of the inkjet ink.

Humectants/Penetrants

If the two or more color inkjet inks of the same color and color densitycontain organic solvents or water, preferably at least one humectant ispresent in the inks to prevent the clogging of the nozzle, due to itsability to slow down the evaporation rate of ink.

Suitable humectants include triacetin, N-methyl-2-pyrrolidone, glycerol,urea, thiourea, ethylene urea, alkyl urea, alkyl thiourea, dialkyl ureaand dialkyl thiourea, diols, including ethanediols, propanediols,propanetriols, butanediols, pentanediols, and hexanediols; glycols,including propylene glycol, polypropylene glycol, ethylene glycol,polyethylene glycol, diethylene glycol, tetraethylene glycol, andmixtures and derivatives thereof. Preferred humectants are triethyleneglycol mono butylether, glycerol and 1,2-hexanediol. The humectant ispreferably added to the inkjet ink formulation in an amount of 0.1 to 40wt % of the formulation, more preferably 0.1 to 10 wt % of theformulation, and most preferably approximately 4.0 to 6.0 wt %.

Other Additives

The two or more color inkjet inks of the inkjet ink set according to thepresent invention may include other additives such as buffering agents,anti-mold agents, pH adjustment agents, electric conductivity adjustmentagents, chelating agents, anti-rusting agents, light stabilizers,dendrimers, polymers, cross-linking agents, soluble electrolytes asconductivity aid, sequestering agents and chelating agents, compounds tointroduce additional security features and the like. Such additives maybe included in the two or more color inkjet inks of the inkjet ink setaccording to the present invention in any effective amount, as desired.

Compounds to introduce additional security features include afluorescent compound, a phosphorescent compound, a thermochromiccompound, an iridescent compound and a magnetic particle. SuitableUV-fluorescent and phosphorescent compounds include LUMILUX™ luminescentpigments from HONEYWELL, UVITEX™ OB from CIBA-GEIGY, KEYFLUOR™ dyes andpigments from KEYSTONE and fluorescent dyes from SYNTHEGEN.

The two or more color inkjet inks of the inkjet ink set according to thepresent invention may further include conducting or semi-conductingpolymers, such as polyanilines, polypyrroles, polythiophenes such aspoly(ethylenedioxythiophene) (PEDOT), substituted or unsubstitutedpoly(phenylenevinylenes) (PPV's) such as PPV and MEH-PPV, polyfluorenessuch as PF6, etc.

Preparation of Pigmented Inkjet Inks

The pigmented inkjet inks be prepared by precipitating or milling thepigment in the dispersion medium in the presence of the polymericdispersant.

Mixing apparatuses may include a pressure kneader, an open kneader, aplanetary mixer, a dissolver, and a Dalton Universal Mixer. Suitablemilling and dispersion apparatuses are a ball mill, a pearl mill, acolloid mill, a high-speed disperser, double rollers, a bead mill, apaint conditioner, and triple rollers. The dispersions may also beprepared using ultrasonic energy.

Many different types of materials may be used as milling media, such asglasses, ceramics, metals, and plastics. In a preferred embodiment, thegrinding media can include particles, preferably substantially sphericalin shape, e.g. beads consisting essentially of a polymeric resin oryttrium stabilized zirconium oxide beads.

In the process of mixing, milling and dispersion, each process isperformed with cooling to prevent build up of heat, and for radiationcurable inkjet inks as much as possible under light conditions in whichactinic radiation has been substantially excluded.

The inkjet ink may contain more than one pigment, the inkjet ink may beprepared using separate dispersions for each pigment, or alternativelyseveral pigments may be mixed and co-milled in preparing the dispersion.

The dispersion process can be carried out in a continuous, batch orsemi-batch mode.

The preferred amounts and ratios of the ingredients of the mill grindwill vary widely depending upon the specific materials and the intendedapplications. The contents of the milling mixture include the mill grindand the milling media. The mill grind includes pigment, polymericdispersant and a liquid carrier. For inkjet inks, the pigment is usuallypresent in the mill grind at 1 to 50 wt %, excluding the milling media.The weight ratio of pigment over polymeric dispersant is 20:1 to 1:2.

The milling time can vary widely and depends upon the pigment,mechanical device and residence conditions selected, the initial anddesired final particle size, etc. In the present invention pigmentdispersions with an average particle size of less than 100 nm may beprepared.

After milling is completed, the milling media is separated from themilled particulate product (in either a dry or liquid dispersion form)using conventional separation techniques, such as by filtration, sievingthrough a mesh screen, and the like. Often the sieve is built into themill, e.g. for a bead mill. The milled pigment concentrate is preferablyseparated from the milling media by filtration.

In general it is desirable to make the inkjet inks in the form of aconcentrated mill grind, which is subsequently diluted to theappropriate concentration for use in the inkjet printing system. Thistechnique permits preparation of a greater quantity of pigmented inkfrom the equipment. By dilution, the inkjet ink is adjusted to thedesired viscosity, surface tension, color, hue, saturation density, andprint area coverage for the particular application.

Spectral Separation Factor

The spectral separation factor SSF was found to be an excellent measureto characterize a pigment inkjet ink, as it takes into accountproperties related to light-absorption (e.g. wavelength of maximumabsorbance λ_(max), shape of the absorption spectrum andabsorbance-value at λ_(max)) as well as properties related to thedispersion quality and stability.

A measurement of the absorbance at a higher wavelength gives anindication on the shape of the absorption spectrum. The dispersionquality can be evaluated based on the phenomenon of light scatteringinduced by solid particles in solutions. When measured in transmission,light scattering in pigment inks may be detected as an increasedabsorbance at higher wavelengths than the absorbance peak of the actualpigment. The dispersion stability can be evaluated by comparing the SSFbefore and after a heat treatment of e.g. a week at 80° C.

The spectral separation factor SSF of the ink is calculated by using thedata of the recorded spectrum of an ink solution or a jetted image on asubstrate and comparing the maximum absorbance to the absorbance at ahigher reference wavelength λ_(ref). The spectral separation factor iscalculated as the ratio of the maximum absorbance A_(max) over theabsorbance A_(ref) at a reference wavelength.

${SSF} = \frac{A_{\max}}{A_{ref}}$

The SSF is an excellent tool to design inkjet ink sets with large colorgamut. Often inkjet ink sets are now commercialized, wherein thedifferent inks are not sufficiently matched with each other. Forexample, the combined absorption of all inks does not give a completeabsorption over the whole visible spectrum, e.g. “gaps” exist betweenthe absorption spectra of the colorants. Another problem is that one inkmight be absorbing in the range of another ink. The resulting colorgamut of these inkjet ink sets is low or mediocre.

EXAMPLES Materials

All materials used in the following examples were readily available fromstandard sources such as Aldrich Chemical Co. (Belgium) and Acros(Belgium) unless otherwise specified. The water used was deionizedwater.

CINQUASIA™ Magenta RT-355-D is a quinacridone pigment from CIBASPECIALTY CHEMICALS.Sunfast™ Blue 15:3 is a cyan pigment (C.I. Pigment Blue 15:3) availablefrom SUN CHEMICAL.SOLSPERSE™ 35000 and SOLSPERSE™ 39000 are polyethyleneimine-polyesterhyperdispersants from NOVEON.SOLSPERSE™ 5000 is a dispersion synergist from NOVEON.GENORAD™ 16 is a polymerization inhibitor from RAHN AG.DPGDA is dipropyleneglycoldiacrylate from SARTOMER.SARTOMER™ SR9003 is a difunctional acrylate monomer available fromSARTOMER.CRAYNOR™ CN 386 is an amine modified acrylate synergist available fromSARTOMER.GENOCURE™ EPD is ethyl 4-dimethylaminobenzoate from RAHN AG.GENOCURE™ TPO is 2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide fromRAHN AG.GENOCURE™ PBZ is 4-phenylbenzophenone, a photo-initiator from RAHN AG.GENOCURE™ ITX is isothioxanthone, a photo-initiator from RAHN AG.BYK™ UV3510 is a polyethermodified polydimethylsiloxane wetting agentfrom BYK CHEMIE GMBH.BYK™-333 is a surfactant from BYK CHEMIE GMBH.Fasson PE85 white/S692N/BG40 is a blown co-extruded, corona-treatedwhite polyethylene substrate from AVERY DENNISON.PE-paper is a polyethylene coated unsubbed RC-paper available from MONDIPACKAGING.Rayoart CGS 92 is a high gloss, two side coated clear, biaxiallyoriented polypropylene film from INNOVIA FILMS.SeeMee Standard Easy is Seemee backlit standard easy, a double sidecoated PVC from VERSEIDAG-INDUTEX GMBH.Fasson MC Primecoat S2000N is FASSON MC Primecoat/S2000N/HF80, a white,one side machine coated, woodfree printing paper substrate from AVERYDENNISON.Oracal 1640 is Oracal Blanc 1640 Print Vinyl, an adhesivepolyvinylchloride substrate from ANTALIS.Biprint 650 gr is Biprint blanc/couleur, a corona treated, polypropyleneboard from ANTALIS.

Measurement Methods 1. Measurement of SSF

The spectral separation factor SSF of an ink was calculated by using thedata of the recorded spectrum of an ink solution and comparing themaximum absorbance to the absorbance at a reference wavelength. Thechoice of this reference wavelength is dependent on the pigment(s) used:

-   -   if the color ink has a maximum absorbance A_(max) between 400        and 500 nm then the absorbance A_(ref) must be determined at a        reference wavelength of 600 nm,    -   If the color ink has a maximum absorbance A_(max) between 500        and 600 nm then the absorbance A_(ref) must be determined at a        reference wavelength of 650 nm,    -   If the color ink has a maximum absorbance A_(max) between 600        and 700 nm then the absorbance A_(ref) must be determined at a        reference wavelength of 830 nm.

The absorbance was determined in transmission with a Shimadzu UV-2101 PCdouble beam-spectrophotometer. The inkjet inks were diluted with ethylacetate to have a pigment concentration according to Table 4.

TABLE 4 InkJet ink with Pigment maximum absorbance A_(max) concentrationbetween 400 and 500 0.002% between 500 and 600 0.005% between 600 and700 0.002%

A spectrophotometric measurement of the UV-VIS-NIR absorption spectrumof the diluted ink was performed in transmission-mode with a doublebeam-spectrophotometer using the settings of Table 5. Quartz cells witha path length of 10 mm were used and ethyl acetate was chosen as ablank.

TABLE 5 Mode Absorbance Wavelength range 240-900 nm   Slit width 2.0 nmScan interval 1.0 nm Scan speed Fast (1165 nm/min) Detectorphoto-multiplier (UV-VIS)

Efficient pigment inkjet inks exhibiting a narrow absorption spectrumand a high maximum absorbance have a value for SSF of at least 30.

2. Average Particle Size

The average particle size of pigment particles in a non-aqueous inkjetink was determined with a Brookhaven Instruments Particle Sizer BI90plusbased upon the principle of dynamic light scattering. The ink ordispersion was diluted with ethyl acetate to a pigment concentration of0.002 wt %. The measurement settings of the BI90plus were: 5 runs at 23°C., angle of 90°, wavelength of 635 nm and graphics=correction function.

For good ink jet characteristics (jetting characteristics and printquality) the average particle size of the dispersed particles should beless than 200 nm, preferably less than 150 nm.

3. Dispersion Stability

The dispersion stability was evaluated by comparing the particle sizebefore and after a heat treatment of 7 days at 83° C. Pigmented inkjetinks exhibiting good dispersion stability have a particle size afterheat treatment which remained below 200 nm, preferably below 150 nm.

4. Surface Tension

The surface tension of the inkjet inks was measured with a KRÜSStensiometer K9 at 25° C. after 60 seconds.

5. Viscosity

The viscosity of the inkjet inks was measured using a Brookfield DV-II+viscometer at 25° C. and shear rate of 4 RPM using a CPE 40 spindle.

6. Dotsize

The dotsize was measured with a CellCheck CNC-SLS (from M-Service &Geräte-Peter Müller, Germany) using a 150× microscope objectiveconnected to a WAT-202B camera (from Watec Co., Ltd. Japan). An averageof 5 dotsize measurements was calculated using the software Metricversie 8.02 Live (from M-Service & Geräte-Peter Müller, Germany).

7. Curing Speed

The percentage of the maximum output of the lamp was taken as a measurefor curing speed, the lower the number the higher curing speed. A samplewas considered as fully cured at the moment scratching with a Q-tipcaused no visual damage.

8. Surface Energy of the Substrate

The Owens-Wendt equation was used for calculating the surface energy ofa substrate σ_(s) in the same manner as disclosed in US 2005190245(AGFA).

9. CIE ΔE2000

The method used is disclosed in the above paragraph “Same color andcolor density”.

Example 1

This example illustrates how the same dotsize was obtained on differentsubstrates using two curable inkjet inks with a different surfacetension.

Preparation of Inkjet Inks

A concentrated pigment dispersion P1 was prepared according to Table 6.

TABLE 6 wt % of: P1 CINQUASIA ™ Magenta RT- 20.0 355-D SOLS PERSE ™35000 20.0 GENORAD ™ 16 1.0 DPGDA 59.0

The concentrated pigment dispersion P1 was made by mixing 360.0 g of thepigment CINQUASIA™ Magenta RT-355-D, 36.0 g of a 50% solution of theinhibitor GENORAD™ 16 in DPGDA and g of a 35% solution of the polymericdispersant SOLSPERSE™ 35000 in DPGDA for 30 minutes using a DISPERLUX™Laboratory Dissolver YELLOW075 from DISPERLUX S.A.R.L., Luxembourg. Themilling mixture was then milled at a rotation speed of 13 m/s and a flowrate of 0.6 L/min. under cooling by a NETZSCH™ LABSTAR1 at a 54.4%volume filling with yttrium-stabilized zirconium oxide-beads of 0.4 mmdiameter (“high wear resistant zirconia grinding media” from TOSOH Co.)and a residence time of 85 minutes. After milling the dispersion wasseparated from the beads using a filter cloth. The concentrated pigmentdispersion P1 had an average particle size of 96 nm and a SSF of 60.

Two curable magenta inkjet inks INK-1 and INK-2 were prepared in thesame manner from the concentrated pigment dispersion P1 by adding theremaining components under stirring at 20° C. to obtain a composition asshown in Table 7.

TABLE 7 wt % of INK-1 INK-2 Dispersion P1 20.00 20.00 DPGDA 59.20 56.20GENOCURE ™ TPO 10.00 10.00 GENOCURE ™ PBZ 5.00 5.00 GENOCURE ™ EPD 5.005.00 GENORAD ™ 16 0.80 0.80 BYK ™ UV 3510 0.00 3.00

The curable inkjet inks INK-1 and INK-2 had a surface tension of 35.1mN/m, respectively 21.8 mN/m at 20° C.

Printing and Evaluation

Table 8 shows the five different substrates and their surface energywhich were selected for this example.

TABLE 8 Surface Substrate Commercial tradename energy (mJ/m²) SUB-1Fasson PE85 38.1 white/S692N/BG40 SUB-2 PE-paper 33.0 SUB-3 Rayoart CGS92 41.8 SUB-4 SeeMee Standard Easy 45.6 SUB-5 Fasson MC Primecoat 46.7S2000N

The five substrates SUB-1 to SUB-5 were printed with the curable inkjetinks INK-1 and INK-2, as well as with an ink-mixture from the curableinkjet inks INK-1 and INK-2 which was selected for having a printdotsize of 110 μm.

The inks were printed with a custom build printer equipped with a fixedUPH™ printhead from AGFA with a distance between nozzle plate andink-receiver of 1.0 mm. The inks were jetted at 5 dpd with a resolutionof 360×360 dpi and in line cured using a 120 W DPL-lamp giving anexposure of 50 W at 400 mm/s. A final curing was given by passing thejetted image twice at 330 mm/s at an exposure of 50 W. Thejet-to-cure-time was 1.3 seconds. The distance between the UV-lamp andthe ink-receiver was 2.2 mm. The jetting temperature was 45° C.

The dotsize obtained for the curable inkjet inks INK-1, INK-2 and theink-mixtures are shown in Table 9.

TABLE 9 Ink-mixture INK-1 INK-2 Surface Dot-size Dot-size wt % wt %tension Dot-size Substrate (μm) (μm) INK-1 INK-2 (mN/m) (μm) SUB-1 113127 99.83 0.17 32.3 111 SUB-2 90 142 96.67 3.33 25.7 110 SUB-3 70 1110.00 100.00 21.8 111 SUB-4 103 153 66.67 33.33 22.4 110 SUB-5 105 12483.33 16.67 23.0 112

From Table 9, it should be clear that a very consistent image qualitywas obtained using ink-mixtures which delivered a dotsize ofapproximately 110 μm compared to prints obtained with one of the curableinkjet inks INK-1 or INK-2. It was noticed that in general the dotsizefrom an ink having no surfactant first decreased slightly by theaddition of a surfactant and then with additional surfactant was added,increased again.

The inkjet ink INK-2 should be selected in function of the desireddotsize and the selected substrates to be printed upon. For example, ifone would like to have a dotsize of 120 μm, this was not possible withINK-2 on substrate SUB-3, unless it was combined with another surfacetreatment. However for the other substrates SUB-4 and SUB-5, requiringalready high amounts of INK-3, it was possible to obtain a dot size of120 μm, since this was inside the range of INK-1 and INK-2. SUB-4 andSUB-5 were printed upon with an ink-mixture according to Table 10.

TABLE 10 Ink-mixture INK-1 INK-2 Surface Dot-size Dot-size wt % wt %tension Dot-size Substrate (μm) (μm) INK-1 INK-2 (mN/m) (μm) SUB-4 103153 33.33 66.67 21.8 120 SUB-5 105 124 66.67 33.33 22.4 119

Another observation is that there is no correlation between the surfaceenergy of a substrate and the surface tension of the ink-mixture forobtaining the same dotsize. This is shown in Table 11.

TABLE 11 Substrate Ink-mixture Surface energy Surface tension Substrate(mJ/m²) (mN/m) SUB-1 38.1 32.3 SUB-2 33.0 25.7 SUB-3 41.8 21.8 SUB-445.6 22.4 SUB-5 46.7 23.0

Frequently commercial substrates are modified in some manner, whichinfluences physical properties of the ink such as spreading andadhesion. Substrates SUB-1 and SUB-2 are both polyethylene substrates,but SUB-2 contains titanium dioxide as a whitening agent. SubstrateSUB-4 had a polymethylmethacrylate coating. Therefore, instead ofmeasuring the surface energy of a substrate and trying to correlate itwith the surface tension of an ink-mixture, it is better to perform adotsize printing test on the substrate with a range of differentink-mixtures.

Example 2

This example illustrates a dotsize printing test on two other substratesSUB-6 and SUB-7 using ink-mixtures of the same curable inkjet inks INK-1and INK-2 of EXAMPLE 1.

Printing and Evaluation

SUB-6 was a PVC substrate for which Oracal 1640 was used, while SUB-7was a polypropylene substrate for which Biprint 650 gr was used. Theresulting dotsize for ink-mixtures of the curable inkjet inks INK-1 andINK-2 printed in the same manner as in EXAMPLE 1 on the substrates SUB-6and SUB-7 is shown in Table 12. The column “wt % of surfactant”corresponds with the amount of the surfactant Byk UV 3510 present in theink-mixture.

TABLE 12 Ink-mixture wt % of wt % of wt % of Dotsize (μm) INK-1 INK-2surfactant SUB-6 SUB-7 100.00 0.00 0.000 95 76 99.83 0.17 0.005 91 6899.67 0.33 0.010 88 71 98.33 1.67 0.050 100 69 96.67 3.33 0.100 134 7083.33 16.67 0.500 162 73 66.67 33.33 1.000 164 87 33.33 66.67 2.000 164158 0.00 100.00 3.000 169 175

From Table 122 it can be seen that for a dotsize of 110 μm on substrateSUB-6, the surfactant concentration of the ink-mixture should be between0.050 and 0.100 wt % based upon the total weight of the ink-mixture. OnSUB-7, the surfactant concentration of the ink-mixture should be between1.000 and 2.000 wt % based upon the total weight of the ink-mixture. Todetermine the exact ink-mixture, a second dotsize printing test can bedone in the narrower range or it can be derived from a graph plottingthe dotsize in function of the surfactant concentration or the amount ofINK-2 in the ink-mixture. For example, the graph for SUB-7 is quitelinear between 1.000 and 2.000 wt % surfactant and it can be derivedthat the surfactant concentration should be about 1.31 wt %, i.e. anink-mixture should be used including 56.33 wt % of INK-1 and 43.67 wt %of INK-2 based upon the total weight of the ink-mixture.

The dotsize data in Table 12 make it also clear that the two substratesreact quite differently on the ink-mixtures. This dotsize data obtainedfor specific ink-mixtures can be stored in a data library for future useof the same substrate. This increases the productivity in an industrialprinting environment since no time must be spent in adjusting ink andsubstrate for the desired spreading of the ink on the substrate whichlargely determines the image quality.

Example 3

This example illustrates how a higher curing speed can be obtained foran ink-mixture of two curable inkjet inks, which would otherwise haveunacceptable dispersion stability if prepared as such.

Preparation of Inkjet Inks

A concentrated pigment dispersion P2 was prepared according to Table 13.

TABLE 13 wt % of: P2 SUNFAST ™ Blue 15:3 14.00 SOLSPERSE ™ 39000 14.00SOLSPERSE ™ 5000 0.75 GENORAD ™ 16 1.0 DPGDA 70.25

The concentrated pigment dispersion P2 was made by mixing 14.00 g of a50% solution of the inhibitor GENORAD™ 16 in DPGDA, 5.25 g of SOLSPERSE™5000, 326.67 g of a 30% solution of the polymeric dispersant SOLSPERSE™39000 in DPGDA and a further 256.08 g of DPGDA for 30 minutes using aDISPERLUX™ Laboratory Dissolver YELLOW075 from DISPERLUX S.A.R.L.,Luxembourg. The milling mixture was then milled under cooling at arotation speed of 4250 RPM by an Eiger bead mill at a 52% volume fillingwith yttrium-stabilized zirconium oxide-beads of 0.4 mm diameter (“highwear resistant zirconia grinding media” from TOSOH Co.) and a residencetime of 25 minutes. After milling the dispersion was separated from thebeads using a filter cloth. The concentrated pigment dispersion P2 hadan average particle size of 103 nm, a viscosity of 53 mPa·s and a SSF of37.

Four curable cyan inkjet inks INK-3 to INK-6 were prepared in the samemanner from the concentrated pigment dispersion P2 to obtain acomposition as shown in Table 14.

TABLE 14 wt % of: Ink-3 Ink-4 Ink-5 Ink-6 Dispersion P2 17.87 17.8717.87 17.87 DPGDA 21.31 28.81 31.31 11.31 SARTOMER ™ SR9003 40.00 40.0040.00 40.00 GENOCURE ™ ITX 5.00 5.00 10.00 — CRAYNOR ™ CN 386 15.00 7.50— 30.00 GENORAD ™ 16 0.82 0.82 0.82 0.82 BYK ™-333 0.03 0.03 0.03 0.03

The curable cyan inkjet inks INK-5 and INK-6 lacked the co-initiatorCRAYNOR™ CN 386, respectively the photo-initiator GENOCURE™ ITX. Anink-mixture MIX-56 was made by mixing the curable cyan inkjet inks INK-5and INK-6 in a wt % ratio of 50:50.

Evaluation of the Inkjet Inks

The dispersion stability of the curable cyan inkjet inks INK-3 to INK-6and the ink-mixture MIX-56 was determined by comparing the averageparticle size after preparation of the ink and again after a heattreatment of 7 days at 83° C. The average particle size of theink-mixture MIX-56 after heat treatment was obtained by mixing theinkjet inks INK-5 and INK-6 in a wt % ratio of 50:50 after they hadreceived a heat treatment of 7 days at 83° C. For good dispersionstability, the average particle size in the ink should remain below 150nm.

TABLE 15 Average particle size (in nm) After After heat Curing InkJetink preparation treatment speed INK-3 106 247 50 INK-4 112 217 100 INK-5110 117 400 INK-6 110 156 >1000 Ink-mixture 106 137 50 MIX-56

The curing speed can be increased by adding more co-initiator CRAYNOR™CN 386, but increasing amounts of the co-initiator also cause thedispersion stability to deteriorate. Table 15 shows that the ink-mixtureMIX-56 exhibited better dispersion stability than the inkjet inks INK-3and INK-4 and at the same time could be cured at very high curing speed.

Example 4

This example illustrates when two inkjet inks are considered to have thesame color and color density.

As already mentioned, two inks are regarded as having different colorand density if CIE ΔE2000 is larger than 5.0. For most inkjetapplications, such as wide format inkjet printing applications,differences in color and density of two inks can be allowable if the CIEΔE2000 value is smaller than or equal to 2.0. The more critical theinkjet application, usually determined by the viewing distance of animage, the smaller the CIE ΔE2000 value should be.

The CIE ΔE2000 was determined for the inks INK-3 to INK-6 of Table 14,whereby the ink INK-3 was used as a first ink, i.e. as a reference. Theinks differ somewhat in composition, but all contain the same amount ofcyan pigment. The values obtained for CIE ΔE2000 in Table 16 give animpression of what can be expected as “normal” ink-production variationsdue to weighing errors, purity of raw materials, etc.

TABLE 16 First Second ink ink CIE ΔE2000 INK-3 INK-4 0.1 INK-3 INK-5 0.3INK-3 INK-6 0.5

In another experiment the cyan ink INK-3 was diluted with DPGDA and theCIE ΔE2000 between INK-3 and the diluted ink was determined. The resultsare given by Table 17.

TABLE 17 % Dilution of INK-3 with DPGDA CIE ΔE2000 0 0.0 5 0.8 10 1.3 151.8 20 2.4 25 3.0

By comparing Table 16 and Table 17, it can be concluded that the“normal” ink-production variations of Table 16 fall well within a 5%margin. For an inkjet application requiring a CIE ΔE2000 smaller than2.0, a dilution of 10% or even 15% may be tolerated for a cyan ink.However, differences in the image due to variation of colorantconcentration can be noticed more easily for a black ink and a magentaink. For the latter two, a dilution of 10% may be seen as an upperlimit. For high quality inkjet printing applications, the mixing errorsof two or more inks should be lower than 5%, i.e. a value for CIE ΔE2000smaller than 1.0.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1-20. (canceled)
 21. An inkjet printing method comprising, in order, thesteps of: a) providing to an inkjet printer at least two or morepigmented color inkjet inks of the same color and color density buthaving a different composition; b) mixing the at least two or more colorinkjet inks in a controlled amount; and c) printing the mixture of theat least two or more color inkjet inks with the inkjet printer onto anink-receiver.
 22. The inkjet printing method according to claim 21,wherein the at least two or more color inkjet inks of the same color andcolor density have a pair-wise color difference CIE ΔE2000 that issmaller than or equal to 2.0.
 23. The inkjet printing method accordingto claim 21, wherein the at least two or more color inkjet inks containthe same colorant or colorants.
 24. The inkjet printing method accordingto claim 21, wherein a surface tension of the at least two or more colorinkjet inks differs by more than 3.0 mN/m.
 25. The inkjet printingmethod according to claim 21, wherein the controlled amount for mixingthe at least two or more color inkjet inks is selected from a printingtest including a number of ink mixtures printed on the ink-receiver. 26.The inkjet printing method according to claim 25, wherein results of theprinting test are stored in a data library.
 27. The inkjet printingmethod according to claim 26, wherein the data library includesinformation on image quality and/or physical properties of ink-mixturesfor a plurality of ink-receivers.
 28. The inkjet printing methodaccording to claim 27, wherein the data library is stored and managed bya computer.
 29. The inkjet printing method according to claim 21,wherein the controlled amount for mixing the at least two or more colorinkjet inks is selected from a data library including information onimage quality and/or physical properties of mixtures of the at least twoor more color inkjet inks on the ink-receiver.
 30. An inkjet ink setcomprising: at least two or more pigmented color inkjet inks of the samecolor and color density; wherein the at least two or more pigmentedcolor inkjet inks have a different composition; and the at least two ormore color inkjet inks contain the same colorant or colorants.
 31. Theinkjet ink set according to claim 30, wherein the at least two or morecolor inkjet inks of the same color and color density have a pair-wisecolor difference CIE ΔE2000 that is smaller than or equal to 2.0. 32.The inkjet ink set according to claim 30, wherein the at least two ormore color inkjet inks include a first ink and a second ink, and asurface tension of the first ink differs by more than 3.0 mN/m from thatof the second ink.
 33. The inkjet ink set according to claim 30, whereinthe at least two or more color inkjet inks include a first ink and asecond ink, and a viscosity of the first ink differs by more than 5.0mPa·s at 30° C. from that of the second ink.
 34. The inkjet ink setaccording to claim 30, wherein the at least two or more color inkjetinks include a first ink and a second ink, and an amount and/or type ofa solvent in the first ink differs from that in the second ink.
 35. Theinkjet ink set according to claim 30, wherein the at least two or morecolor inkjet inks include a first ink and a second ink, and an amountand/or type of a monomer in the first ink differs from that in thesecond ink.
 36. The inkjet ink set according to claim 30, wherein the atleast two or more color inkjet inks include a first ink and a secondink, and an amount and/or type of an initiator in the first ink differsfrom that in the second ink.
 37. The inkjet ink set according to claim30, wherein the at least two or more color inkjet inks include a firstink and a second ink, and a volume of the first ink differs from that ofthe second ink.
 38. An inkjet printer comprising: the inkjet ink set asdefined by claim
 30. 39. The inkjet printer according to claim 38,further comprising: a device arranged to evaluate or measure propertiesrelated to print quality of the ink set.
 40. The inkjet printeraccording to claim 39, wherein the device arranged to evaluate ormeasure properties related to the print quality is incorporated in theinkjet printer.